bugfix: case-insensitive usernames, reject non-positive bookmark page

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>

backend/Cargo.lock

@@ -1033,7 +1033,7 @@ checksum = "5e5032e24019045c762d3c0f28f5b6b8bbf38563a65908389bf7978758920897"
 
 [[package]]
 name = "mangalord"
-version = "0.9.2"
+version = "0.9.3"
 dependencies = [
  "anyhow",
  "argon2",