fix(backend): make company research notes per user

[CMaintz]

Company research notes were stored on the shared companies row, so every user read and overwrote the same notes. Worse, CompanyGroundingService fed those notes into cover-letter prompts, so my notes could end up grounding someone else's letter.

Stacked on #160 because the Testcontainers base only exists there so far.

• V036 adds user_company_notes, keyed by (user_id, company_id), and drops the two columns from companies.
• Existing notes have no author, so I give a note to a user only when exactly one user has an application or a tracked outreach for that company. If several users do, or nobody does, the note is dropped. Copying it to everyone involved would just keep the leak. In practice that's only been me so far, so nothing real should get lost.
• New CompanyResearchRepositoryPort + adapter. The use case, controller and grounding service all take the current user's id now.
• Tests: two-user cases in CompanyServiceTest and a new CompanyGroundingServiceTest, Postgres tests for the adapter, and a migration test that runs V035, seeds old-style notes, then applies V036 and checks where each one ended up.
• PostgresIntegrationTest now shares one container across the run. With one container per class, the second class with the same Spring config got the cached context pointing at a stopped database. The new migration test hit that.

The API and frontend are unchanged. 571 backend tests pass locally with Docker.