🤖 ci(docker): auto-build on push to main + fix root Dockerfile swag step

Enable Docker image build on every push to main (paths-ignore for docs-only changes mirrors webapp pattern). Fix root Dockerfile (shipped by #89) which was missing the swag init step required for //go:embed of swagger.json (gitignored).

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

Dockerfile

@@ -14,6 +14,13 @@ RUN go mod download
 # Copy entire source code
 COPY . .
 
+# Generate Swagger documentation if not already present
+# (pkg/server/docs/ is gitignored ; the binary //go:embed depends on it)
+RUN if [ ! -f pkg/server/docs/swagger.json ]; then \
+      go install github.com/swaggo/swag/cmd/swag@latest && \
+      cd pkg/server && go generate ; \
+    fi
+
 # Build the server binary
 RUN go build -o app ./cmd/server