Engram v1.10.8 — FTS5 Topic Search Fix

Fixes mem_search not finding observations by topic_key, and adds a direct search fallback for edge cases where FTS5 tokenization misses results.

Bug Fix

topic_key added to FTS5 index — searching for topic keys like architecture/auth-model now returns matches via full-text search
Direct search fallback — if FTS5 returns zero results, a LIKE-based fallback query runs against key text fields to catch cases where FTS5 tokenization fails (e.g., slash-separated keys, special characters)