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>

scripts/generate_master_key.py

@@ -2,7 +2,7 @@
 
 Usage:
     python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"
-    # put that value in XPRV_ENCRYPTION_KEY (.env), then:
+    # write it to the file XPRV_ENCRYPTION_KEY_FILE (.env) points at, then:
     python scripts/generate_master_key.py
 """
 
@@ -12,7 +12,7 @@ from app.wallet.verify import load_sample
 
 if __name__ == "__main__":
     if not settings.xprv_encryption_key:
-        raise SystemExit("XPRV_ENCRYPTION_KEY is not set")
+        raise SystemExit("XPRV_ENCRYPTION_KEY_FILE is not set, or its file is missing or empty")
     sample = load_sample()
     if sample.users or sample.rounds or sample.seeds:
         # SEC-50: a new key cannot reproduce the addresses the database already holds.