feat: daily encrypted database backup to a bucket of its own

A backup service runs pg_dump every day at 03:00 Brasília, checks the archive,
encrypts it with age to a public key and uploads it with a token for that
bucket only. The server cannot read or delete backups: the private key stays
with the owner, the bucket's lifecycle rule expires copies and its lock stops
early deletion. Each run is recorded and shown on the Kanban's Integrations
tab. tests/backup_test.py backs up, restores into a scratch database and
compares the rows in CI. Setup and restore: docs/BACKUP.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitea/workflows/deploy.yml

@@ -93,6 +93,7 @@ jobs:
           $COMPOSE exec -T api python -m tests.retention_test
           $COMPOSE exec -T api python -m tests.runtime_security_test
           $COMPOSE exec -T api python -m tests.tiny_oauth_test
+          $COMPOSE exec -T backup python -m tests.backup_test
 
       # Run Chrome on the Compose network. It must resolve the same storage:9000
       # hostname used in presigned URLs, and absence of Chrome must fail CI.
@@ -106,7 +107,7 @@ jobs:
         if: failure()
         run: |
           $COMPOSE ps || true
-          $COMPOSE logs --tail 200 api worker site kanban || true
+          $COMPOSE logs --tail 200 api worker backup site kanban || true
 
       - name: Tear down
         if: always()