Feature: AdSense Ads System (FEAT-41)

Metadata

Implements a controlled ads layer for StudyBoost using Google AdSense with strict gating rules:

system-level gate via FEAT-35 feature flag enable_ads
user-level gate via FEAT-39 subscription state (free users only; active paid users excluded)

The integration keeps ads non-intrusive, fails silently when ad loading fails, and avoids security-sensitive routes.

Adds a reusable ads module:
- frontend/components/ads/ads-provider.tsx
- frontend/components/ads/use-ads-eligibility.ts
- frontend/components/ads/ad-slot.tsx
Loads AdSense script lazily only when all eligibility checks pass.
Hides ads while subscription/flag state is loading.
Hides ads if:
- feature flag is disabled/missing
- user is active paid (pro / plus)
- AdSense env config is missing
- provider script fails or is blocked
Renders conservative placements on:
- home discovery page
- courses index
- course detail
- document preview/detail

Extends FEAT-35 known flag enum with:
- FeatureFlagName.ENABLE_ADS = 'enable_ads'
Reuses existing GET /feature-flags/status/:name endpoint for runtime ads gating.
No new Ads-specific endpoint added.
No new database table or migration added for v1.

Scenario ID	Description	Steps	Input	Expected Result
FEAT-41-01	Free-user ad render	Enable flag, set AdSense env, open approved route	Free user	Ad slot appears in approved section
FEAT-41-02	Paid-user suppression	Enable flag, open approved route as active paid user	Active `pro` / `plus`	No ad slot and no ad-script usage for rendering
FEAT-41-03	Flag off global suppression	Disable `enable_ads`, open approved route	Any user	No ads rendered
FEAT-41-04	Provider blocked/failure	Block AdSense script/ad network	Eligible free user	UI remains stable; no runtime crash
FEAT-41-05	Loading-state suppression	Delay subscription/flag response	Eligible route	Ads hidden until eligibility is resolved

Scoped FEAT-41 backend lint and backend build passed.
Scoped FEAT-41 frontend lint passed.
QA bug fix: ads now remain hidden until the current route/config pair has its own resolved enable_ads status, preventing stale flag reuse after route/config transitions.
Full frontend production build is currently blocked by an upstream prerender dependency unrelated to ads: GET /documents?page=1&limit=3&sort=trending returns 500 from the backend.