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/compose.yaml

@@ -45,10 +45,11 @@ services:
     volumes:
       # SQLite database, pictures and APKs: a bind-mounted folder on the Mac (git-ignored).
       - ${KVITT_DEV_DATA_DIR:-./data}:/data
-      # The secrets folder, read-only, so the server finds /secrets/fcm-service-account.json (git-ignored).
-      # The folder is mounted rather than the file: a missing source file would make Docker create a directory
-      # in its place, while a missing key must simply leave push disabled with a start-up warning (§10.4).
-      - ${KVITT_SECRETS_DIR:-../secrets}:/secrets:ro
+      # Only the FCM key, read-only (D-117): up.sh copies ${KVITT_SECRETS_DIR}/fcm-service-account.json into
+      # infra/dev/secrets/ (git-ignored), which holds nothing else. A folder is mounted rather than the file, because
+      # a missing source file would make Docker create a directory in its place, while a missing key must simply
+      # leave push disabled with a start-up warning (§10.4); without the key the folder is empty.
+      - ./secrets:/secrets:ro
     healthcheck:
       test: ["CMD", "kvitt-server", "healthcheck"]
       interval: 30s