feat: rebuild MoonCode as a SaaS analysis platform

[fuchencong]

Summary

• rebuild MoonCode around repositories, bare Git mirrors, immutable commit snapshots, and snapshot-bound analysis runs
• add personal GitHub PAT provider connections independent from MoonCode workspace authorization
• integrate Tinyauth through a same-origin authentication flow and Hatchet-backed durable workflows
• reorganize the Go backend into domain-oriented biz/data/workflow layers backed by PostgreSQL and sqlc
• add SCC code-scale analysis, Feishu/DingTalk channels, retention workflows, audit records, OpenAPI, and operational metrics
• redesign the frontend workbench for repositories, snapshots, analyses, channels, credentials, and workspace administration
• add CI, release automation, deployment images, vulnerability scanning, secret scanning, and SBOM generation

Validation

• make test vet lint frontend-i18n frontend-lint frontend-build
• make integration-test against PostgreSQL 17 with per-test schema isolation
• make generate/sqlc and go mod tidy
• govulncheck v1.1.4: no reachable vulnerabilities
• npm audit --audit-level=high: 0 vulnerabilities
• gitleaks v8.28.0: no leaks found
• backend and frontend deployment images build successfully
• git diff --check