Infra security fixes: Caddy log redaction, headers and non-root container; dev mounts only the FCM key; deploy records image digests (D-117)

- Caddyfile: access and error logs replace the /i/<code> segment and every query string with REDACTED
  (regexp filter on request>uri); Authorization stays redacted by default. Deferred response headers:
  Content-Security-Policy, X-Content-Type-Options, X-Frame-Options, Referrer-Policy.
- Pi compose: caddy as uid 10002 with group 1000, read-only root, cap_drop ALL plus NET_BIND_SERVICE
  (needed to exec the image's file-capability binary), tmpfs /config and /tmp; install.sh sets /certs
  0750 1000:1000 and Caddy's /data to 10002.
- deploy.sh and install.sh print the pulled image digests and append them to /opt/kvitt/deployed-digests,
  warning when a recorded tag changed digest; the workflow prints the pushed digests; kvitt_cicd needs
  only write:package.
- Dev stack: up.sh stages only fcm-service-account.json into infra/dev/secrets/ (git-ignored), mounted
  read-only at /secrets.
- README: restricted cert-sync key required (plain root key not acceptable), remote_ip and log checks,
  APK download note, owner decisions (backup encryption, APK limit, key form, token scope), overhaul lines.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B

infra/dev/up.sh

@@ -21,11 +21,8 @@ fi
 export KVITT_PUBLIC_URL="$public_url"
 
 data_dir="$(dev_path "$(env_value KVITT_DEV_DATA_DIR ./data)")"
-secrets_dir="$(dev_path "$(env_value KVITT_SECRETS_DIR ../secrets)")"
 mkdir -p "$data_dir"
-if [[ ! -f "$secrets_dir/fcm-service-account.json" ]]; then
-  echo "up.sh: no $secrets_dir/fcm-service-account.json; the server starts with push disabled." >&2
-fi
+stage_fcm_key
 
 compose up -d server