test: add a real-Postgres harness for end-to-end suites in tests/

[TheGreatAxios]

Summary

• tests/lib/db-harness.ts creates a fresh artifact_<random>_test database for each suite on the ARTIFACT_DATABASE_URL server. It applies Interchange's runMigrations and then runArtifactMigrations, and drops the database afterwards. The same destructive-test gate as the src suite guards it. artifactApp mounts createArtifactRoutes for a seeded tenant principal, authorized by the real createRequireGrant over the database's grant table. bun run test runs src/ and tests/.
• New e2e suites:
  • An upload, revise and download round trip on a filesystem ContentStore (tests/lib/fs-content-store.ts).
  • Upload policy: an oversize file gets 413, a disallowed MIME type gets 415, and neither leaves an artifact behind.
  • The migration contract: the schema and tables exist, and applying twice changes nothing and keeps rows. This replaces src/migrations.test.ts.
• Deleted destructive-tests-gate, counts and preview unit tests. A route test now covers the counts walk across more than one page.
• Deleted artifacts.test.ts cases that mount.test.ts already covers through routes.
• The reference-host acceptance suite moves to tests/reference-host.test.ts and runs in bun run test. The test:acceptance script and its CI steps are gone.
  • The example is no longer its own package. The root tsconfig maps @corbits/artifacts to src/, and the example shares the test database URL helpers.
  • Its Interchange wiring now matches the 0.4.0 types.
  • @intx/hub-common and @intx/hub-sessions are now pinned devDependencies.
  • This also makes CL-9332's lockfile refresh moot, since the example's lockfile is gone.
• On this branch stored upload bytes are not versioned (fix(uploads): store upload bytes per version #31 adds that), so version 2 in the round trip is a metadata-only revise that downloads the same bytes, and the test asserts that ?version=1 returns 400.
• Stacked on feat(routes)!: replace mountArtifacts with createArtifactRoutes #28.

Verification

• bun install --frozen-lockfile, typecheck clean
• 334 tests pass (src + tests/, including the 42 acceptance scenarios) against Postgres 16, with the coverage floor met
• build

Closes CL-9324
Closes CL-9149
Closes CL-9154
Closes CL-9140
Closes CL-9133
Closes CL-9136
Closes CL-9145

[TheGreatAxios]

Reviewed all seven commits over three review passes. The harness gets a fresh database per suite through the destructive-test gate and uses the real grant evaluator. The round trip, the 413/415 checks and the migration tests go through the mounted app on real Postgres. The counts test seeds 120 rows, so it really crosses a page boundary. Each deletion matches a case the route suite already covers, and the acceptance suite now runs in bun run test. No open findings.