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

Dockerfile

@@ -1,15 +1,14 @@
 # Build stage
 FROM golang:1.24-alpine AS builder
-RUN apk add --no-cache gcc musl-dev
 WORKDIR /app
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
-RUN CGO_ENABLED=1 go build -o pr-helper .
+RUN CGO_ENABLED=0 go build -o pr-helper .
 
 # Runtime stage
 FROM alpine:3.20
-RUN apk add --no-cache ca-certificates
+RUN apk add --no-cache ca-certificates tzdata
 WORKDIR /app
 COPY --from=builder /app/pr-helper .
 COPY --from=builder /app/templates ./templates