feat: add Docker infrastructure, migrations, CI/CD client, session cleanup, tests

Docker & Deployment:
- Add Dockerfile (multi-stage, alpine, non-root)
- Add docker-compose.yml (caddy, backend, postgres, watchtower)
- Add Caddyfile (TLS, file_server, reverse proxy)
- Add .env.example

Database:
- Add migrations/001_init.sql (all tables + indexes)

CI/CD:
- Add cmd/ci-release/main.go (launcher binary upload tool)

Session management:
- Add internal/session/cleanup.go (background expired session cleanup)
- Integrate cleanup worker into main.go

Bug fixes:
- Fix launcherLatest download URL to include version segment
- Fix serveLauncherAsset path to match route pattern
- Add Content-Type detection from file extension in CAS serveFile
- Add empty-field validation in webLogin
- Format string fix in ci-release (%d → %s for resp.Status)

Tests:
- Add internal/auth/auth_test.go (8 tests)
- Add internal/cas/cas_test.go (7 tests)
- Add internal/session/cleanup_test.go (1 test)
- Add internal/api/api_test.go (5 tests)
- All tests passing, go vet clean

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/session/cleanup_test.go

@@ -0,0 +1,14 @@
+package session
+
+import (
+	"testing"
+	"time"
+)
+
+func TestStartCleanupWorker(t *testing.T) {
+	// Verify the worker starts and can be cancelled without panic.
+	cancel := StartCleanupWorker(nil, 1*time.Millisecond)
+	defer cancel()
+	// Give it a moment to attempt one cleanup cycle.
+	time.Sleep(50 * time.Millisecond)
+}