Replace gorm.io/driver/sqlite (mattn/go-sqlite3, CGO) with
github.com/glebarez/sqlite (modernc, pure Go). Builds and tests no
longer need a C toolchain, fixing CI on runners without gcc. Drop the
race detector from CI (it requires CGO) and build a static binary in
Docker.
Multi-stage Dockerfile builds the frontend and backend into a single
Alpine image bundling Caddy, started via an entrypoint. CI runs backend
tests (with race detector), builds the frontend and publishes the image
to GHCR on master/tags. Targets a self-hosted runner with a local build
cache. Updates .gitignore (secrets/runtime state) and README.
