Persist URL dedup cache to SQLite (bounded, ~KB); keep video blobs in memory

Switch storage backend from in-memory to SQLite (silences the library warning).
recent_urls (the edit-redelivery dedup map) now survives restarts: it's tiny and
TTL-bounded, stored via the library KV store, with wall-clock timestamps and
prune-on-load. last_video is deliberately NOT persisted — each value is a base64
video up to ~133 MB, so persisting it would bloat the DB; it stays in memory.
DB lives at bot-state.db (gitignored).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -8,6 +8,10 @@ signal-cli-data.bak.*
 # Local service file (contains your phone number) — use signal-bot.service.example
 signal-bot.service
 
+# Runtime state (SQLite dedup cache) — created next to bot.py at runtime
+bot-state.db
+bot-state.db-*
+
 # Python
 venv/
 __pycache__/