feat(database): support MySQL as the business database

[WHUTcjh-2024]

Closes #1418

Summary

• Support MySQL 8.0.16+ as the primary business database while preserving PostgreSQL and SQLite behavior.
• Add a version-74 MySQL schema baseline and the required SQL-dialect adaptations for repositories, JSON operations, locking, and task claiming.
• Keep vector retrieval on an external vector-capable backend, with fail-fast validation for incompatible configurations.
• Add a Docker Compose override, migration-head drift protection, and production recovery guidance.

Validation

• go test ./internal/database ./internal/application/repository ./internal/container
• go test ./internal/application/service
• go vet ./internal/database ./internal/application/repository ./internal/application/service ./internal/container
• MySQL 8.0.37 and 8.4.10 integration: special-character credentials, UTC sessions, migrate up/down/up, 50-table schema parity, migration-version lookup, soft-delete uniqueness, dirty-state fail-fast, and live repository queries
• docker compose -f docker-compose.yml -f docker-compose.mysql.yml --profile qdrant config --quiet

[WHUTcjh-2024]

已解决与最新 main 的冲突并推送：补齐 MySQL 75–78 迁移，并适配新增的 metadata 前缀查询。已通过相关 Go 包测试、MySQL 8.4 迁移 up/down/up 回环及仓储集成测试。