feat(bot): unified topic processor replacing router+interpreter stack

Replace 3-layer architecture (gpt-5-nano router + gpt-4o-mini interpreter) with
single unified topic processor (gpt-4o-mini) for simplified message handling.

New components:
- HouseholdContextCache: TTL-based caching (5 min) for household config data
- TopicProcessor: Unified classification + parsing with structured JSON output

Key changes:
- Renamed ASSISTANT_ROUTER_MODEL → TOPIC_PROCESSOR_MODEL
- Added TOPIC_PROCESSOR_TIMEOUT_MS (default 10s)
- Refactored save() → saveWithInterpretation() for pre-parsed interpretations
- Removed deprecated createOpenAiTopicMessageRouter and ~300 lines legacy code
- Fixed typing indicator to only start when needed (purchase routes)
- Fixed amount formatting: convert minor units to major for rawText

Routes: silent, chat_reply, purchase, purchase_clarification, payment,
payment_clarification, topic_helper, dismiss_workflow

All 212 bot tests pass. Typecheck, lint, format, build clean.

.env.example

@@ -16,9 +16,10 @@ MINI_APP_ALLOWED_ORIGINS=http://localhost:5173
 
 # Parsing / AI
 OPENAI_API_KEY=your-openai-api-key
-PARSER_MODEL=gpt-4o-mini
 PURCHASE_PARSER_MODEL=gpt-4o-mini
 ASSISTANT_MODEL=gpt-4o-mini
+TOPIC_PROCESSOR_MODEL=gpt-4o-mini
+TOPIC_PROCESSOR_TIMEOUT_MS=10000
 ASSISTANT_TIMEOUT_MS=20000
 ASSISTANT_MEMORY_MAX_TURNS=12
 ASSISTANT_RATE_LIMIT_BURST=5