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)
 # ---------------------------------------------------------------------------