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/repos.go

@@ -104,7 +104,7 @@ func (h *ReposHandler) CleanupRepos(c *gin.Context) {
 	if maxAgeDays == "" {
 		maxAgeDays = "7"
 	}
-	rows, err := h.db.Query(`SELECT id, local_path FROM repositories WHERE user_id = ? AND last_used < datetime('now', '-' || ? || ' days')`, user.ID, maxAgeDays)
+	rows, err := h.db.Query(`SELECT id, local_path FROM repositories WHERE user_id = ? AND last_used < DATE_SUB(NOW(), INTERVAL ? DAY)`, user.ID, maxAgeDays)
 	if err != nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 		return
@@ -244,7 +244,7 @@ func (h *ReposHandler) GetGraph(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)
 
 	repo, err := services.OpenRepo(localPath)
 	if err != nil {
@@ -297,7 +297,7 @@ func (h *ReposHandler) GetDiff(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)
 
 	repo, err := services.OpenRepo(localPath)
 	if err != nil {