feat: migrate database from SQLite to MySQL with .env configuration

- Replace go-sqlite3 with go-sql-driver/mysql (pure Go, no CGO needed)
- Add joho/godotenv for .env file loading
- Rewrite database/db.go with MySQL-compatible DDL (BIGINT AUTO_INCREMENT, InnoDB, utf8mb4)
- Convert all SQLite-specific SQL: INSERT OR IGNORE → INSERT IGNORE,
  INSERT OR REPLACE → INSERT ... ON DUPLICATE KEY UPDATE,
  datetime('now') → NOW(), date arithmetic → DATE_SUB()
- Add MySQL config fields to config.go (host/port/user/password/database)
- Add .env.example with connection parameter template
- Update Dockerfile: remove CGO/gcc/musl dependency, smaller build
- Update docker-compose.yml: add MySQL service container with healthcheck
- Update CLAUDE.md documentation

handlers/generate.go

@@ -73,7 +73,7 @@ func (h *GenerateHandler) Generate(c *gin.Context) {
 	}
 
 	// Update last_used
-	h.db.Exec(`UPDATE repositories SET last_used = datetime('now') WHERE id = ?`, id)
+	h.db.Exec(`UPDATE repositories SET last_used = NOW() WHERE id = ?`, id)
 
 	// Generate PR description (pass user ID for per-user LLM config)
 	pr, err := services.GeneratePR(h.db, localPath, req.Base, req.Head, user.ID, sendEvent)