refactor(dal): clarify token uniqueness — UNIQUE constraint on tokens.user_name + SingleOrDefaultAsync

[Vasar007]

Captured as a phase-02 review follow-up (WR-01 finding in .planning/phases/02-test-coverage/02-REVIEW.md and 02-REVIEWS.md). Backlog Phase 999.3 in ROADMAP.md.

Problem

DatabaseRefreshTokenInfoService.FindByUserIdAsync (Sources/Libraries/ProjectV.DataAccessLayer/Services/Tokens/DatabaseRefreshTokenInfoService.cs:68) uses FirstOrDefaultAsync(token => token.UserId == rawUserId) while the parallel FindByUserNameAsync in DatabaseUserInfoService uses SingleOrDefaultAsync. The raw-SQL schema bootstrap in DbCollectionFixture.ApplySchemaAsync creates the tokens table without a UNIQUE constraint on user_name (which stores the user's Guid).

Two scenarios:

1. If the design intent is one token per user (the production token-issuance flow deletes the old token before inserting a new one), the schema should enforce that invariant with UNIQUE and the service should use SingleOrDefaultAsync so a duplicate-row bug surfaces immediately.
2. If the design intent is multiple tokens per user (sliding window), the service should document it and add a test for the multi-token scenario.

The current state is asymmetric: enforcement is policy-only (service layer), and the integration tests do not exercise the duplicate-row case.

Acceptance Criteria

• Decide between Option A (UNIQUE + Single) and Option B (document multi-token intent + add test)
• Apply the chosen direction in production migration + the test schema in DbCollectionFixture
• Add an integration test that exercises whichever invariant is chosen

Why deferred

This is an architectural decision requiring stakeholder input on token lifecycle policy — out of scope for Phase 2 (Test Coverage). The pre-existing production behavior (FirstOrDefaultAsync) is unchanged by Phase 2; this issue captures the gap for explicit resolution in a future milestone.

Related

• Phase 2 review: .planning/phases/02-test-coverage/02-REVIEW.md (WR-01)
• Phase 2 review summary: .planning/phases/02-test-coverage/02-REVIEWS.md
• Originally surfaced during 02-09 (dal-integration-testcontainers) plan execution