feat(sp12): SQLCipher encryption at rest (optional)

- New cyclone.db_crypto module:
  * is_sqlcipher_available() — capability check
  * is_encryption_enabled() — Keychain key + sqlcipher3 present
  * get_db_key() — reads 'cyclone.db.key' from Keychain
  * make_sqlcipher_connect_creator(url, key) — SQLAlchemy creator
- db._make_engine() now switches to SQLCipher when key is present
- pyproject.toml: optional 'sqlcipher' extra (sqlcipher3>=0.6,<1)
- Fallback: without Keychain key, DB stays plain SQLite (no surprise
  behavior for operators who haven't set up encryption yet)
- Verified: encrypted file is unreadable as plain SQLite, wrong key
  raises on first query, migrations + ORM work transparently
- HIPAA §164.312(a)(2)(iv) compliance note in docs

Tests: 705 -> 717 (12 new for SQLCipher). All 717 backend tests pass.

docs/reference/co-medicaid.md

@@ -147,6 +147,50 @@ All CMS POS codes `01`–`99` are accepted. The canonical list lives in
 `cyclone/parsers/payer.py` as `CMS_PLACE_OF_SERVICE_CODES` and is the
 source of truth for validation and any UI dropdowns.
 
+## Database encryption at rest (SP12)
+
+Cyclone optionally encrypts the SQLite database with SQLCipher
+(AES-256). The encryption key is stored in macOS Keychain — never on
+disk in plaintext. Without the key, the DB falls back to plain SQLite.
+
+### One-time operator setup
+
+```bash
+# 1. Install SQLCipher (C library) and the Python binding.
+brew install sqlcipher
+pip install -e backend[sqlcipher]
+
+# 2. Generate a random 32-byte key and store it in Keychain.
+python3 -c "import secrets; print(secrets.token_urlsafe(32))" \
+    | xargs -I {} security add-generic-password \
+        -s cyclone -a cyclone.db.key -w "{}"
+
+# 3. Restart Cyclone. The DB is now encrypted.
+```
+
+### Verification
+
+```bash
+# The DB file should be unreadable as plain SQLite.
+sqlite3 ~/.local/share/cyclone/cyclone.db "SELECT count(*) FROM sqlite_master"
+# → file is not a database
+
+# But readable through Cyclone.
+curl http://localhost:8000/api/claims
+# → 200 OK
+```
+
+### Key rotation (future)
+
+To rotate the key:
+
+1. Decrypt with old key, dump to SQL
+2. Re-encrypt with new key, import SQL
+3. Update Keychain
+
+A first-class rotation endpoint is out of scope for SP12 (planned for
+SP14+).
+
 ## Audit log (SP11)
 
 Cyclone persists every state-changing event to a tamper-evident