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

CLAUDE.md

@@ -8,7 +8,7 @@ PR-Helper is a self-hosted web service that auto-generates PR descriptions from
 
 ## Tech Stack
 
-- **Backend**: Go + Gin + SQLite (go-sqlite3) + go-git
+- **Backend**: Go + Gin + MySQL + go-git
 - **Frontend**: Go html/template + HTMX + D3.js + diff2html + Tailwind CSS
 - **LLM**: OpenAI-compatible API (SSE streaming)
 - **Deploy**: Docker
@@ -16,14 +16,17 @@ PR-Helper is a self-hosted web service that auto-generates PR descriptions from
 ## Build & Run
 
 ```bash
-# Build (CGO required for SQLite)
-CGO_ENABLED=1 go build -o pr-helper .
+# Build
+go build -o pr-helper .
 
-# Run
+# Configure — copy .env.example to .env and set MySQL credentials
+cp .env.example .env
+
+# Run (requires MySQL server)
 ./pr-helper
 # Server listens on :8080
 
-# Docker
+# Docker (includes MySQL container)
 docker compose up --build
 ```
 
@@ -33,13 +36,13 @@ docker compose up --build
 handlers/   → HTTP handlers (pages + JSON API + SSE endpoints)
 services/   → Business logic (git ops, LLM calls, PDF gen, cache management)
 models/     → Data models (repository, settings, analysis)
-database/   → SQLite init and migrations
+database/   → MySQL init and migrations
 config/     → Configuration loading
 templates/  → Go HTML templates (layouts/, pages/, partials/)
 static/     → CSS (Tailwind output), JS (graph, diff-viewer, sse), vendor libs
 ```
 
-**Data flow**: Browser ↔ Gin handlers → services (git/llm) → SQLite + filesystem (`data/`)
+**Data flow**: Browser ↔ Gin handlers → services (git/llm) → MySQL + filesystem (`data/`)
 
 **Key service layer responsibilities**:
 - `services/git.go` — clone, diff, graph data extraction via go-git
@@ -63,8 +66,9 @@ static/     → CSS (Tailwind output), JS (graph, diff-viewer, sse), vendor libs
 
 ## Data Storage
 
-- SQLite DB at `data/pr-helper.db` — settings (KV), repositories, analyses, review_notes
+- MySQL database `pr_helper` — settings (KV), repositories, analyses, review_notes
 - Cloned repos cached at `data/repos/` with configurable expiry (default 7 days)
+- Database connection configured via `.env` file (see `.env.example`)
 
 ## Development Notes