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/tests/test_acks.py

@@ -51,19 +51,19 @@ def test_migration_0002_creates_acks_table():
 
 def test_migration_latest_idempotent_on_fresh_db():
     """Re-running the migration on the same DB must be a no-op (PRAGMA
-    user_version already at the latest version — currently 11 after
+    user_version already at the latest version — currently 12 after
     0004-0006 line_reconciliation, 0005 ta1_acks, SP9's 0007
     providers/payers/clearhouse, SP10's 0008 payer_rejected,
     SP11's 0009 audit_log, SP14's 0010 payer_rejected_acknowledged,
-    SP16's 0011 processed_inbound_files)."""
+    SP16's 0011 processed_inbound_files, SP17's 0012 db_backups)."""
     with db.engine().begin() as c:
         v1 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v1 == 11
+    assert v1 == 12
     # A second run should not raise and should not bump the version.
     db_migrate.run(db.engine())
     with db.engine().begin() as c:
         v2 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v2 == 11
+    assert v2 == 12
 
 
 def test_add_ack_persists_row():