Replace ChromaDB with pgvector for vector storage

Consolidate onto PostgreSQL by using pgvector instead of a separate
ChromaDB instance. This removes a Docker volume, a large dependency,
and simplifies the stack without meaningful performance impact at
our document scale.

- Swap langchain-chroma for langchain-postgres (PGVector)
- Use pgvector/pgvector:pg16 Docker image with init script
- Lazy-initialize vector store to avoid eager DB connections
- Add SQL helpers for stats/delete/list (replacing _collection access)
- Remove legacy main.py, chunker, petmd scraper, and /api/query endpoint

Re-index required after deploy (POST /api/rag/index + /index-obsidian).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

blueprints/conversation/agents.py

@@ -328,7 +328,7 @@ async def obsidian_search_notes(query: str) -> str:
         return "Obsidian integration is not configured. Please set OBSIDIAN_VAULT_PATH environment variable."
 
     try:
-        # Query ChromaDB for obsidian documents
+        # Query vector store for obsidian documents
         serialized, docs = await query_vector_store(query=query)
         return serialized