fix(db): fix migration failures when upgrading from 0.8.0 schema

Migration 1: remove idx_sessions_token_hash from _SCHEMA_STATEMENTS.
The legacy schema has sessions.token (not token_hash). The IF NOT EXISTS
guard only prevents duplicate index names — it still requires the column
to exist. Migration 2 drops and rebuilds sessions with token_hash anyway,
so creating the index in migration 1 was redundant.

Migration 3: replace ALTER TABLE ADD COLUMN with a table rebuild.
SQLite rejects ALTER TABLE ADD COLUMN NOT NULL DEFAULT <expression> when
the table already contains rows. The old DB has ~181k geo_cache rows, so
the ALTER always failed. Rebuild copies existing rows with last_seen set
to cached_at as a reasonable approximation of last-seen time.

backend/pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "bangui-backend"
-version = "0.9.19-rc.1"
+version = "0.9.19-rc.3"
 description = "BanGUI backend — fail2ban web management interface"
 requires-python = ">=3.12"
 dependencies = [