replace static translation files with LLM-based i18n

Instead of maintaining a YAML file per language, German (de.yaml) is the
single source of truth. For any other language, the label strings are
translated on demand via an LLM call and cached in memory — no static
files to maintain, any language the parent writes in is served
automatically.

- Add src/notifications/i18n.py: get_strings(), _translate() via
  litellm.completion, in-memory cache, clear_cache() for tests
- Passthrough keys (reg_fee_amount, deposit_amount) are never sent to
  the LLM so currency amounts are guaranteed to be unchanged
- Falls back to German silently if the LLM call fails
- Remove src/notifications/i18n/en.yaml (no longer needed)
- Remove load_strings() from context.py (moved to i18n.py)
- Add model parameter to AdminNotifier (defaults to claude-haiku)
- Add TestGetStrings suite covering: no LLM for German, LLM called for
  others, caching, fallback, passthrough key preservation
- Add autouse reset_translation_cache fixture to isolate tests

https://claude.ai/code/session_01LjjK7RjKVnC8bETtccfgna

src/notifications/context.py

@@ -1,9 +1,6 @@
 """Pure functions for building email context dicts from registration data."""
 
 from datetime import date, datetime
-from pathlib import Path
-
-import yaml
 
 from ..models.registration import RegistrationData
 
@@ -17,22 +14,6 @@ QR_STREET = "Huebwisstrase 5"
 QR_PCODE = "8117"
 QR_CITY = "Fällanden"
 
-_I18N_DIR = Path(__file__).parent / "i18n"
-
-# ---------------------------------------------------------------------------
-# i18n
-# ---------------------------------------------------------------------------
-
-
-def load_strings(language: str) -> dict:
-    """Load the label/string table for *language* (falls back to German)."""
-    locale_file = _I18N_DIR / f"{language}.yaml"
-    if not locale_file.exists():
-        locale_file = _I18N_DIR / "de.yaml"
-    with locale_file.open(encoding="utf-8") as fh:
-        return yaml.safe_load(fh)
-
-
 # ---------------------------------------------------------------------------
 # Formatting helpers (pure functions, no side-effects)
 # ---------------------------------------------------------------------------

src/notifications/i18n.py

@@ -0,0 +1,103 @@
+"""i18n support for parent confirmation emails.
+
+German is the canonical source language (de.yaml).  For any other language
+the German labels are translated on-demand via an LLM call and cached
+in-process for the lifetime of the server — no static translation files to
+maintain, any language the parent writes in is supported automatically.
+"""
+
+import json
+import logging
+from pathlib import Path
+
+import litellm
+import yaml
+
+logger = logging.getLogger(__name__)
+
+_I18N_DIR = Path(__file__).parent / "i18n"
+
+# In-memory translation cache keyed by language code.
+_cache: dict[str, dict] = {}
+
+# Pure data values that must never be sent to the LLM for translation.
+_PASSTHROUGH_KEYS = {"reg_fee_amount", "deposit_amount"}
+
+_SYSTEM_PROMPT = """\
+You are a translation assistant for a Swiss playgroup registration system.
+Translate the following JSON label strings from German into {language}.
+
+Rules:
+- Return ONLY a valid JSON object with the exact same keys and structure.
+- Preserve all {{placeholder}} variables exactly as-is (e.g. {{name}}).
+- Preserve all HTML tags and entities exactly (e.g. <strong>, &nbsp;).
+- Keep proper nouns untranslated: "Spielgruppe Pumuckl", "Familienverein Fällanden".
+- Do not include any explanation or text outside the JSON."""
+
+
+def get_strings(language: str, model: str) -> dict:
+    """Return the label string table for *language*.
+
+    For German, loads directly from de.yaml (no LLM call).
+    For all other languages, translates the German labels via LLM and caches
+    the result in memory.  Falls back to German if the LLM call fails.
+    """
+    if language == "de":
+        return _load_german()
+
+    if language in _cache:
+        return _cache[language]
+
+    german = _load_german()
+    translated = _translate(german, language, model)
+    _cache[language] = translated
+    return translated
+
+
+def clear_cache() -> None:
+    """Evict all cached translations (intended for use in tests)."""
+    _cache.clear()
+
+
+def _load_german() -> dict:
+    with (_I18N_DIR / "de.yaml").open(encoding="utf-8") as fh:
+        return yaml.safe_load(fh)
+
+
+def _translate(german: dict, language: str, model: str) -> dict:
+    """Translate the German label dict into *language* via LLM.
+
+    Returns the German dict unchanged if the LLM call fails or returns
+    malformed JSON.
+    """
+    passthrough = {k: german[k] for k in _PASSTHROUGH_KEYS if k in german}
+    to_translate = {k: v for k, v in german.items() if k not in _PASSTHROUGH_KEYS}
+
+    system = _SYSTEM_PROMPT.format(language=language)
+    payload = json.dumps(to_translate, ensure_ascii=False, indent=2)
+
+    try:
+        response = litellm.completion(
+            model=model,
+            messages=[
+                {"role": "system", "content": system},
+                {"role": "user", "content": payload},
+            ],
+            max_tokens=2048,
+        )
+        raw = response.choices[0].message.content.strip()
+
+        # Strip markdown code fences that some models add
+        if raw.startswith("```"):
+            raw = raw[raw.index("\n") + 1 :]
+            raw = raw[: raw.rfind("```")]
+
+        translated: dict = json.loads(raw)
+        translated.update(passthrough)
+        return translated
+
+    except Exception:
+        logger.exception(
+            "Failed to translate email labels into %s — falling back to German", language
+        )
+        return german

src/notifications/i18n/en.yaml

@@ -1,57 +0,0 @@
-subject: "Registration Confirmation – Spielgruppe Pumuckl"
-greeting: "Dear {name}"
-intro: >-
-  Your registration with Spielgruppe Pumuckl has been received.
-  Here is a summary:
-
-child_section: "Child Details"
-child_name: "Name"
-child_dob: "Date of birth"
-child_needs: "Special needs"
-
-booking_section: "Playgroup Booking"
-booking_type: "Type"
-booking_days: "Days"
-
-fees_section: "Fees"
-monthly_fee: "Monthly subscription"
-reg_fee: "Registration fee (one-time, first year)"
-reg_fee_amount: "CHF 80.00"
-deposit: "Cleaning deposit – indoor (refundable)"
-deposit_amount: "CHF 50.00"
-
-parent_section: "Your Contact Details"
-parent_name: "Name"
-parent_address: "Address"
-parent_phone: "Phone"
-parent_email: "Email"
-
-emergency_section: "Emergency Contact"
-emergency_name: "Name"
-emergency_phone: "Phone"
-
-payment_section: "Payment Details"
-payment_intro: >-
-  Please transfer the registration fee of <strong>CHF&nbsp;80.00</strong> to the account below.
-  You can scan the QR code with your banking app:
-payment_intro_text: "Please transfer the registration fee of CHF 80.00 to the following account:"
-iban_label: "IBAN"
-payee_label: "Payee"
-amount_label: "Amount"
-
-closing: |
-  If you have any questions, we are happy to help. We look forward to welcoming your child!
-
-  Kind regards
-  Spielgruppe Pumuckl
-
-none: "None"
-
-days:
-  monday: "Monday"
-  wednesday: "Wednesday"
-  thursday: "Thursday"
-
-types:
-  indoor: "Indoor Playgroup"
-  outdoor: "Forest Playgroup"

src/notifications/notifier.py

@@ -23,8 +23,8 @@ from .context import (
     build_admin_update_context,
     build_parent_context,
     format_types,
-    load_strings,
 )
+from .i18n import get_strings
 from .renderer import render_template
 
 logger = logging.getLogger(__name__)
@@ -51,6 +51,7 @@ class AdminNotifier:
         indoor_email: str = "",
         outdoor_email: str = "",
         cc_emails: list[str] | None = None,
+        model: str = "anthropic/claude-haiku-4-5-20251001",
     ) -> None:
         self._smtp_host = smtp_host
         self._smtp_port = smtp_port
@@ -61,6 +62,7 @@ class AdminNotifier:
         self._indoor_email = indoor_email
         self._outdoor_email = outdoor_email
         self._cc_emails: list[str] = cc_emails or []
+        self._model = model
 
     # ------------------------------------------------------------------
     # Public API
@@ -140,7 +142,7 @@ class AdminNotifier:
             logger.warning("No parent email in registration — confirmation not sent.")
             return
 
-        strings = load_strings(language)
+        strings = get_strings(language, self._model)
 
         try:
             qr_png = self._generate_qr_bill_png()