feat(sp17): encrypted DB backup automation

Adds automated encrypted backups of the live SQLite file. Closes the
'no backup automation' gap called out in the completeness review
(docs/reviews/2026-06-20-cyclone-completeness-review.md §3.1 #3) and
gives the SP16 MFT scheduler a recovery path when the MFT pipeline
loses days of inbound 999/277CA work in a single crash.

Architecture
------------
- AES-256-GCM with PBKDF2-HMAC-SHA256 (200,000 iters, 16-byte salt)
- Online backups via SQLite's .backup() API — no app downtime
- Salt + passphrase persisted to macOS Keychain (separate accounts
  backup.passphrase + backup.salt) so the key is reproducible across
  processes
- Two-step restore (initiate → confirm) with a one-shot 64-char hex
  token; the second call disposes + rebuilds the engine only if the
  token matches within a 5-minute TTL
- Tamper-evident audit chain (SP11) — db.backup_created,
  db.backup_failed, db.backup_pruned, db.backup_restored,
  db.backup_passphrase_set
- BackupService + BackupScheduler + module-level singletons
- 8 admin endpoints + 6 CLI subcommands
- Auto-start opt-in via CYCLONE_BACKUP_AUTOSTART=true; default
  interval 24h, default retention 30 days
- Fallback posture: if no separate passphrase is set and SQLCipher
  is enabled, the key is derived from the SQLCipher DB key with a
  fixed salt + WARNING log (degraded but never plaintext)

New modules
-----------
- cyclone.backup          — PBKDF2, AES-GCM, sidecar format
- cyclone.backup_service  — create_now / list / verify / restore / prune / status
- cyclone.backup_scheduler — async tick loop with audit hooks

New surface
-----------
- 8 admin endpoints under /api/admin/backup/*
- 6 CLI subcommands under cyclone backup (init-passphrase, create,
  list, verify, restore, prune, status)
- Migration 0012_backups.sql + DbBackup ORM
- store.add_backup_pending()

Tests
-----
- 14 unit tests in test_backup_crypto.py (key derivation, encrypt/
  decrypt round-trip, tampered ciphertext, wrong passphrase, sidecar
  round-trip, filename format)
- 19 tests in test_backup_service.py (create/list/verify/restore/
  prune/status, error handling, fallback key, module singleton)
- 14 API tests in test_api_backup.py (all 8 endpoints + scheduler
  endpoints, two-step restore, error responses)
- 10 tests in test_backup_scheduler.py (tick / start / stop /
  audit / coalescing / module singleton)
- 5 CLI tests in test_cli_backup.py (create / list / verify /
  restore confirm prompt / prune confirm prompt /
  init-passphrase minimum-length check)

Total new tests: 62. All pass. Full backend suite: 833 passed,
9 skipped (gitignored prodfiles), 1 warning.

Design doc: docs/superpowers/specs/2026-06-21-cyclone-encrypted-backup-design.md
README: new 'Encrypted Backups (SP17)' section, SP17 entry in
Roadmap, retention default documented in §Project layout.

backend/src/cyclone/store.py

@@ -1849,6 +1849,32 @@ class CycloneStore:
         with db.SessionLocal()() as s:
             return s.get(db.Two77caAck, ack_id)
 
+    # -- SP17: encrypted DB backups -------------------------------------
+
+    def add_backup_pending(self, *, filename: str, backup_dir: str) -> db.DbBackup:
+        """Insert a ``pending`` row for a backup that is about to start.
+
+        The BackupService fills in ``status`` / ``size_bytes`` /
+        ``db_fingerprint`` / ``table_count`` / ``completed_at`` after
+        the encrypted blob lands on disk.
+        """
+        with db.SessionLocal()() as s:
+            row = db.DbBackup(
+                filename=filename,
+                backup_dir=backup_dir,
+                size_bytes=0,
+                db_fingerprint=None,
+                table_count=0,
+                created_at=utcnow(),
+                completed_at=None,
+                status="pending",
+                error_message=None,
+            )
+            s.add(row)
+            s.commit()
+            s.refresh(row)
+            return row
+
     # -- manual reconciliation (T12) -----------------------------------
 
     def list_unmatched(self, *, kind: str = "both") -> dict: