FTS5-only fallback when Qdrant unavailable

**Epic**: #364

## Description

When Qdrant is unavailable (not configured or connection failed), fall back to FTS5-only keyword search instead of returning empty results.

## Tasks

- [ ] Detect Qdrant availability at startup
- [ ] `SemanticMemory::recall()` uses FTS5-only path when Qdrant down
- [ ] Log warning on fallback
- [ ] Unit test with mocked Qdrant failure