fix: make project deletion tolerant of stale session state

[tamish560]

What

Makes project deletion (DELETE /api/v1/projects/{id}) robust to stale, legacy, or inconsistent session state by making session teardown errors non-fatal during project removal.

Why

When a project has stale sessions (locked worktrees, archived project refs, missing runtime handles, etc.), TeardownProject returns an error and the entire Remove call fails with INTERNAL_ERROR. The project stays registered and the user has no recovery path from the UI.

The issue (#2598) explicitly says: "local project removal must be robust to stale, legacy, partially missing, or otherwise inconsistent AO-managed state. If the user asks to remove a local project registration, AO should work around bad session/worktree/runtime records and complete the unregister flow wherever it can do so without deleting unpurged session state."

Closes #2598

How

Changed Service.Remove in backend/internal/service/project/service.go to ignore the error from m.sessions.TeardownProject(ctx, id) instead of propagating it. The project is still archived (soft-deleted) regardless of whether session teardown succeeded.

This means:

• Sessions that can be torn down, will be
• Sessions that cannot (stale, locked, missing) are left for later cleanup
• The project is always archived, so the user can re-add it without a blocked delete

Testing

Updated TestManager_RemoveDoesNotArchiveWhenTeardownFails to TestManager_RemoveToleratesTeardownFailure. The test now verifies that:

• Remove succeeds (no error) even when teardown fails
• The project is archived (no longer active) after Remove returns

Run: cd backend && go test ./internal/service/project/...

[tamish560]

Closing this in favor of #2599 by @AgentWrapper which addresses the same issue (#2598). Should have checked for existing PRs before opening this one.