fix(admin): bound audit log memory retention and add pagination (closes #926)

[mopxam-art]

Why

src/lib/audit.ts declared an unbounded in-memory array store that grew on every POST /admin/update-scores invocation without any retention cap or eviction. Furthermore, GET /admin/audit returned the entire filtered array without pagination support, leading to unbounded memory growth and large payload responses.

What

• Added bounded in-memory retention cap in src/lib/audit.ts (MAX_AUDIT_LOG_ENTRIES, default 1,000, configurable via environment variable) with automatic FIFO eviction when capacity is exceeded.
• Added helper functions setMaxAuditLogEntries, getMaxAuditLogEntries, and getAuditStoreSize for runtime configuration and memory inspection.
• Added query parameter pagination (limit clamped between 1 and 100, default 50; offset, default 0) to GET /admin/audit in src/routes/admin.ts.
• Retained backwards-compatible response metadata including total, count, limit, offset, and has_more.
• Documented pagination query parameters and memory retention in API.md.
• Added comprehensive unit and integration test suite (src/__tests__/audit-ring-buffer.test.ts) covering boundary limits, filtering combinations, CSV export, validation, and high-throughput memory stability.

Verified

• All unit tests pass (jest src/__tests__/audit-ring-buffer.test.ts, 54 tests pass).
• Full repository test suite passes (123/123 test suites, 1,473 tests pass).
• TypeScript typecheck (npx tsc --noEmit) and linter (npm run lint) clean with 0 errors.
• Production build (npm run build) succeeds cleanly.

Closes #926