No pagination strategy for `getMostSolvedPuzzles`

[mercy60]

Internal ID: #84

Imported from ISSUES.md via scripts/import-issues.mjs.

Problem Statement. Returns full list. With 1000s of puzzles, a single response could be huge.

Expected Outcome.

• ?limit=10&offset=0 query params.

Acceptance Criteria.

• Load test asserts response < 50KB.

Files / modules affected.

• backend/src/analytics/analytics.controller.ts

Dependencies. None.

Difficulty / Effort. Easy / S (≈ 2 hours).

Labels. area:backend, performance