Epic: M19 — Hybrid memory search

[bug-ops]

Motivation

Zeph relies solely on Qdrant vector search for semantic recall. Hybrid FTS5 + vector search with weighted merge significantly improves recall quality, especially for exact term matches and structured queries.

Scope

1. FTS5 virtual table in existing SQLite database for keyword search
2. Hybrid retriever combining Qdrant vector + SQLite FTS5 results with configurable weights
3. Graceful degradation — FTS5-only mode when Qdrant unavailable

Issues

• Add FTS5 virtual table for keyword search in SQLite #372 Add FTS5 virtual table for keyword search in SQLite
• Implement hybrid retriever (vector + keyword merge) #373 Implement hybrid retriever (vector + keyword merge)
• FTS5-only fallback when Qdrant unavailable #374 FTS5-only fallback when Qdrant unavailable