Harden the container: non-root, pinned images and dependencies, the key in a file (B-91, SEC-51, SEC-52, SEC-53; closes B-71, B-72)

- app runs as uid 1000; a one-shot data-permissions service chowns ./data to it
  and closes it to others (works on an older root-owned install with no host sudo);
  files are created 0600 (umask 077)
- python:3.12-slim and caddy:2-alpine pinned by digest; dependencies installed from
  requirements.lock with --require-hashes
- the Fernet key is XPRV_ENCRYPTION_KEY_FILE (./secrets/xprv_encryption.key, a compose
  secret), out of every environment variable and of a ./data backup
- setup.md no longer presents a second way to run the server (B-72)

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

app/wallet/verify.py

@@ -115,7 +115,7 @@ def verify_master_key() -> None:
     except Exception as exc:
         raise ConfigError(
             f"the master key at {settings.master_key_path} cannot be loaded ({type(exc).__name__}): "
-            "check MASTER_KEY_PATH and XPRV_ENCRYPTION_KEY"
+            "check MASTER_KEY_PATH and XPRV_ENCRYPTION_KEY_FILE"
         ) from exc
     problems = sample_problems(account, load_sample(), settings.xprv_encryption_key)
     if problems: