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.go

@@ -0,0 +1,68 @@
+// package session manages Yggdrasil session lifecycle.
+
+package session
+
+import (
+	"context"
+	"log"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"gitea.mrixs.me/Mrixs/MrixsCraft-server/internal/database"
+)
+
+// StartCleanupWorker launches a background goroutine that deletes expired
+// yggdrasil_sessions every interval. It stops when the context is cancelled.
+func StartCleanupWorker(db *database.DB, interval time.Duration) context.CancelFunc {
+	ctx, cancel := context.WithCancel(context.Background())
+
+	go func() {
+		log.Printf("Session cleanup worker started (interval: %v)", interval)
+		ticker := time.NewTicker(interval)
+		defer ticker.Stop()
+
+		// Run once on start.
+		cleanup(db)
+
+		for {
+			select {
+			case <-ticker.C:
+				cleanup(db)
+			case <-ctx.Done():
+				log.Println("Session cleanup worker stopped")
+				return
+			}
+		}
+	}()
+
+	return cancel
+}
+
+func cleanup(db *database.DB) {
+	if db == nil {
+		return
+	}
+	pool := db.Pool()
+	if pool == nil {
+		return
+	}
+	tag, err := pool.Exec(context.Background(),
+		`DELETE FROM yggdrasil_sessions WHERE expires_at < NOW()`)
+	if err != nil {
+		log.Printf("Session cleanup error: %v", err)
+		return
+	}
+	if tag.RowsAffected() > 0 {
+		log.Printf("Session cleanup: removed %d expired sessions", tag.RowsAffected())
+	}
+}
+
+// CountActive returns the number of non-expired sessions.
+func CountActive(pool *pgxpool.Pool) (int, error) {
+	var count int
+	err := pool.QueryRow(context.Background(),
+		`SELECT COUNT(*) FROM yggdrasil_sessions WHERE expires_at > NOW()`,
+	).Scan(&count)
+	return count, err
+}