Two related correctness fixes from the audit:
- Username uniqueness was case-sensitive (`username text UNIQUE`), so
"Alice" and "alice" could both register and then race on login.
Migration 0006 adds a unique index on `lower(username)`; the
existing constraint is kept (overlapping but cheap) to avoid a
destructive migration on any deployments that may already exist.
`repo::user::find_by_username` now matches on `lower(username) =
lower($1)` so login is case-insensitive against the same index.
Test: registering "alice" then "Alice" returns 409 conflict; login
with "ALICE" succeeds against the existing user.
- `POST /api/v1/bookmarks` silently accepted `page: 0` and `page: -1`
even though both are nonsense for a 1-indexed page number. Reject
with 422 `validation_failed` and `details.page` populated, matching
the pattern used for missing-metadata / empty-title elsewhere. Test
covers both 0 and -1.
Lockstep version bump to 0.9.4.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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