RAG Semantic Search Exists in Code But Has No API/UI Integration

[krishnasahoo11156]

embedEmail() and searchSimilarEmails() are implemented in ai.service.ts and test-embeddings.ts,
but there is NO way for users to actually use semantic search. The search bar falls back to PostgreSQL
ILIKE pattern matching.

Steps to Reproduce:

1. Open frontend inbox
2. Type “DBMS project deadline” in search
3. Observe: only exact/partial text matches returned, not semantically similar emails

Expected Behavior:
Search should use pgvector cosine similarity to find emails with related meaning even if keywords don’t
match exactly.

Fix Required:
• Expose POST /api/rag/search endpoint
• Add semantic search toggle to frontend search bar
• Ensure pgvector extension is enabled
• Background job to index historical emails