feat(stalk): remove dead mobile-legends, free-fire, tiktok, instagram

Audit live-probed all 8 stalk upstreams today. 4 alive (github, youtube,
genshin enka, twitter fxtwitter). 4 dead with no free alternative reachable
from VPS: mobile-legends (gempaytopup /stalk-ml now 405), free-fire
(discordbot.freefirecommunity dead 404), tiktok (WAF -> login page, no
rehydration data), instagram (mollygram captcha; i.instagram requires
login/rate-limited 401). All free alternates probed dead (onrender FF 404,
freefireapi.me 404, i.instagram 401 require_login, m.tiktok 403).

Keep 4 alive: github, youtube, genshin, twitter. stalk.rs now clippy-clean.

src/observability/openapi_modules.rs

@@ -74,12 +74,8 @@ use utoipa::OpenApi;
         // ── Stalk module ───────────────────────────────────────────
         crate::presentation::handler::stalk::github_handler,
         crate::presentation::handler::stalk::youtube_handler,
-        crate::presentation::handler::stalk::mobile_legends_handler,
-        crate::presentation::handler::stalk::free_fire_handler,
         crate::presentation::handler::stalk::genshin_handler,
         crate::presentation::handler::stalk::twitter_handler,
-        crate::presentation::handler::stalk::tiktok_handler,
-        crate::presentation::handler::stalk::instagram_handler,
         // ── Search module ──────────────────────────────────────────
         crate::presentation::handler::search::bmkg_handler,
         crate::presentation::handler::search::jadwal_sholat_handler,
@@ -109,7 +105,7 @@ use utoipa::OpenApi;
         (name = "komik", description = "Komik endpoints"),
         (name = "download", description = "Media download endpoints (23 platforms)"),
         (name = "misc", description = "Miscellaneous utilities (currency, gold, server)"),
-        (name = "stalk", description = "User profile stalking (GitHub, YT, Twitter, TikTok, IG, FF, ML, Genshin)"),
+        (name = "stalk", description = "User profile stalking (GitHub, YT, Twitter, Genshin)"),
         (name = "search", description = "Search endpoints (BMKG, prayer times, weather, YouTube, Google)"),
         (name = "tool", description = "Utility tools (WHOIS, IP lookup, URL shortener, tracking, hosting)"),
         (name = "image", description = "Image generation (brat static & animated)"),