feat: inject today's date into system prompt and add extended thinking support

Fixes age validation errors caused by the LLM not knowing the current date.

Changes:
- prompts.py: inject date.today() at the top of both system prompts so the
  LLM can accurately calculate a child's age from their date of birth
- llm.py: add optional thinking_budget parameter to complete(); when set,
  passes thinking={"type": "enabled", "budget_tokens": N} to litellm and
  raises max_tokens to thinking_budget + 4096 (Anthropic models only)
- config.py: add thinking_budget field, read from THINKING_BUDGET env var
- .env.example: document the THINKING_BUDGET option
- core.py: pass thinking_budget through to llm.complete()
- main.py: pass thinking_budget when constructing EmailAgent
- chat_app.py: switch from stream_complete to asyncio.to_thread(complete)
  so extended thinking works and so only the reply field is shown to
  the parent (not the raw JSON wrapper)

To enable extended thinking set THINKING_BUDGET=8000 in .env.

https://claude.ai/code/session_01SUWzMzFvSfWiHXA2p6rPg9

src/agent/prompts.py

@@ -1,6 +1,7 @@
 """Build the system prompt sent to the LLM on every turn."""
 
 import json
+from datetime import date
 
 from ..knowledge_base.loader import KnowledgeBase
 from ..models.conversation import ConversationState
@@ -154,8 +155,12 @@ def _build_registration_prompt(kb: KnowledgeBase, state: ConversationState) -> s
     reg_json = json.dumps(state.registration.to_dict(), ensure_ascii=False, indent=2)
     step_hint = STEP_DESCRIPTIONS.get(state.flow_step, "Continue the conversation.")
 
+    today = date.today().isoformat()
+
     return f"""You are the registration assistant for Spielgruppe Pumuckl, run by Familienverein Fällanden in Fällanden, Switzerland. You help parents register their children for the playgroup and answer questions about it.
 
+**Today's date is {today}.** Use this exact date when calculating a child's age from their date of birth.
+
 {_PERSONALITY}
 
 ## Registration Flow (8 steps)
@@ -200,8 +205,12 @@ def _build_post_completion_prompt(kb: KnowledgeBase, state: ConversationState) -
     reg_json = json.dumps(state.registration.to_dict(), ensure_ascii=False, indent=2)
     child_name = state.registration.child.full_name or "their child"
 
+    today = date.today().isoformat()
+
     return f"""You are the registration assistant for Spielgruppe Pumuckl, run by Familienverein Fällanden in Fällanden, Switzerland.
 
+**Today's date is {today}.**
+
 {_PERSONALITY}
 
 ## Context: Registration Already Complete