Add Dockerfile, fix SQLite immutable mode, URL-encode credentials

- Dockerfile: multi-stage build with golang:1.26 and alpine + ffmpeg
- SQLite: use file: URI with immutable=1 for read-only access
- URL builder: encode user/pass with PathEscape/QueryEscape for
  special characters (@, \, :, etc.)
- Health endpoint: truncate uptime to seconds
- Release skill: update smoke test to /api endpoint
- Remove unused ValidateID function

.claude/skills/release_strix/SKILL.md

@@ -121,7 +121,7 @@ Verify the new version tag exists and both amd64 and arm64 platforms are present
 ```bash
 docker run --rm -d --name strix-smoke-test -p 14567:4567 eduard256/strix:$VERSION
 sleep 5
-curl -s http://localhost:14567/api/v1/health | jq '.version'
+curl -s http://localhost:14567/api | jq '.version'
 docker stop strix-smoke-test
 ```