feat: cursor pagination /api/users

[dragespips]

Implements GET /api/users with keyset pagination over (created_at, id) for stable, efficient listing of all registered users.

Changes:

• src/services/userService.ts: add listUsers() + UserListRow type
  • Keyset WHERE: (created_at < cursorTime) OR (created_at = cursorTime AND id < cursorId)
  • Sort: DESC created_at, DESC id
  • Fetch limit+1 to detect next page without a COUNT query
  • Reuses encodeCursor/decodeCursor from src/utils/cursor.ts
• src/routes/users.ts: add GET / handler
  • zod validation for cursor (optional) and limit (1–100, default 20)
  • Returns { data: UserListRow[], nextCursor: string | null }
  • Invalid/tampered cursor forwarded to service (graceful restart from page 1)
• src/openapi/registry.ts: register GET /api/users (operationId: listUsers)
  • Documents cursor, limit query params and UserListRow schema
• tests/usersList.test.ts: 14 focused unit tests (no live DB)
  • Happy paths, limit validation, cursor threading, error propagation

closes #408

[greatest0fallt1me]

Merged via direct push to main (admin)