Add cursor pagination

[gloskull]

Motivation

• Implement cursor/keyset pagination for GET /api/users/:address/predictions so large result sets can be streamed efficiently and deterministically.
• Avoid skipping rows when multiple predictions share the same createdAt timestamp.
• Validate incoming cursor values to prevent malformed cursors from reaching the service layer.

Description

• Added strict cursor validation on the route using a regex for the createdAt|id form and preserved limit coercion in src/routes/users.ts with zod parsing.
• Implemented stable keyset pagination in src/services/userService.ts by splitting the cursor into createdAt and id and applying WHERE (createdAt < cursorTime) OR (createdAt = cursorTime AND id < cursorId) to match the ORDER BY createdAt DESC, id DESC ordering.
• Continued returning nextCursor as an ISO timestamp plus prediction id (<createdAt.toISOString()>|<id>) from the last item on the page.
• Added focused tests in tests/users.test.ts that seed deterministic createdAt values and IDs, assert pagination across tied timestamps does not skip rows, and verify malformed cursors are rejected.

Testing

• Ran npx eslint src/routes/users.ts src/services/userService.ts which passed for the modified files.
• Ran npm test -- --runInBand tests/users.test.ts which could not complete due to unavailable local services (Postgres/Redis connection refused), so tests were not executed successfully in this environment.
• Ran npm run lint and npm run build which failed due to pre-existing lint/type errors outside the scope of these changes; these failures are unrelated to the cursor pagination logic.

Closes #209