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/src/repo/user.rs

@@ -28,9 +28,17 @@ pub async fn create(pool: &PgPool, username: &str, password_hash: &str) -> AppRe
     }
 }
 
+/// Case-insensitive lookup so login with "Alice" matches a user
+/// registered as "alice" (the unique index on `lower(username)` keeps
+/// the comparison cheap). Equivalent in spirit to ILIKE but uses the
+/// functional index directly.
 pub async fn find_by_username(pool: &PgPool, username: &str) -> AppResult<Option<User>> {
     let row = sqlx::query_as::<_, User>(
-        r#"SELECT id, username, password_hash, created_at FROM users WHERE username = $1"#,
+        r#"
+        SELECT id, username, password_hash, created_at
+        FROM users
+        WHERE lower(username) = lower($1)
+        "#,
     )
     .bind(username)
     .fetch_optional(pool)