feat: add high-value backend modules

[im10furry]

Summary

• add three registry modules: migrations, metrics, and problem
• update registry index to 0.3.0, docs navigation, README/AGENTS module counts, and CI/release module lists
• regenerate embedded registry bundle and harden review-found edge cases around migration path validation, DB row caps, response writer behavior, and panic metrics/problem responses

Validation

• go test -count=1 ./...
• go vet ./...
• all 15 registry modules: go test + go vet (auth uses -timeout=5m because bcrypt cost-boundary test is intentionally slow)
• go run ./internal/cmd/build-bundle
• go run ./cmd/scion doctor --strict
• cd docs && npm run build
• CLI smoke with a built scion.exe: list, info metrics, add migrations, add metrics --standalone, add problem