feat: add SQLite storage for login attempts and sessions

Adds persistent storage using modernc.org/sqlite (pure Go). Login
attempts are deduplicated by (username, password, ip) with counts.
Sessions and session logs are tracked with UUID IDs. Includes embedded
SQL migrations, configurable retention with background pruning, and
an in-memory store for tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -34,6 +34,9 @@ Key settings:
 - `auth.accept_after` — accept login after N failures per IP (default `10`)
 - `auth.credential_ttl` — how long to remember accepted credentials (default `24h`)
 - `auth.static_credentials` — always-accepted username/password pairs
+- `storage.db_path` — SQLite database path (default `oubliette.db`)
+- `storage.retention_days` — auto-prune records older than N days (default `90`)
+- `storage.retention_interval` — how often to run retention (default `1h`)
 
 ### Run