ci: align Gitea CI/CD + Docker with the notify-bridge template

Adopt the proven notify-bridge pipeline pattern and fix deployment bugs.

Workflows:
- build.yml: split into parallel frontend / backend / build-image jobs.
  Run svelte-check + vitest + `go vet ./...` + `go test ./internal/...`
  (tests were never executed in CI). Use buildx with GHA layer cache and
  pin Go to 1.25. Quote the `if:` skip-guard so it is valid YAML.
- release.yml: gate the release on a passing test job, then build & push
  the image, then create the Gitea release LAST so a failed image build
  can no longer leave an orphan release. Use buildx + registry buildcache,
  a hard registry login (a push failure now fails the release), and
  auto-generate a changelog between tags.

Docker:
- Dockerfile: pin golang to 1.25 (matches go.mod's `go 1.25.0`), add
  BuildKit cache mounts for the module + build caches, an OCI source
  label, VOLUME /app/data, and a HEALTHCHECK on /readyz.
- docker-compose.yml: fix the healthcheck — it targeted POST-only
  /api/auth/login (405 -> always unhealthy); now /readyz. Point the image
  name at the Gitea registry tag with build-from-source as the default.
- .dockerignore: exclude ~95 MB of stray binaries, logs, env, and CI/doc
  files from the build context.

docker-compose.yml

@@ -1,7 +1,13 @@
 services:
   tinyforge:
+    # Default: build from source so a fresh clone works out of the box.
     build: .
-    image: tinyforge:latest
+    # Image name doubles as the Gitea registry tag. To DEPLOY the pre-built
+    # image instead of building (e.g. Portainer pulling on a webhook), comment
+    # out `build:` above — compose will then pull this tag. `:latest` is pushed
+    # only for stable (non pre-release) releases, and the registry may require
+    # `docker login git.dolgolyov-family.by` first if the package is private.
+    image: git.dolgolyov-family.by/alexei.dolgolyov/tiny-forge:latest
     container_name: tinyforge
     restart: unless-stopped
     ports:
@@ -31,7 +37,10 @@ services:
     networks:
       - staging-net
     healthcheck:
-      test: ["CMD", "wget", "--no-verbose", "--tries=1", "--spider", "http://localhost:8080/api/auth/login"]
+      # /readyz is the public readiness probe (pings the DB, rate-limited).
+      # The previous target (/api/auth/login) is POST-only, so a GET/spider
+      # request returned 405 and the container was always reported unhealthy.
+      test: ["CMD", "wget", "--no-verbose", "--tries=1", "--spider", "http://localhost:8080/readyz"]
       interval: 30s
       timeout: 5s
       retries: 3