Video Engine Generate

Growth OS Phase 7 -- on-demand "Generate Now" trigger for the admin panel, so this can be tested/verified before wiring any automatic daily schedule. Gated by the video_engine flag (default OFF) on top of the admin-token check -- this is the heaviest single operation in Growth OS (TTS calls + ffmpeg render), so it must never run from a stale/cached admin.html session if the feature's been intentionally turned off. Runs synchronously and returns the result dict as-is (including a graceful {"available": False, "message": ...} if TTS or ffmpeg isn't actually configured on this deploy).

2026-08-04 v2: lang/aspect_ratio/theme are now real choices (see services/video_engine_service.py's _SCRIPT/_ASPECT_RATIOS/_THEMES). post_to_telegram defaults to False and is an explicit per-click admin opt-in, not automatic -- repeatedly clicking Generate Now while testing shouldn't quietly spam a live public channel every time; the admin has to tick the box each time they actually want that.

2026-10-01 (AJ: "加可選GEN不同資產，佢日日都一樣果3隻" -- the daily video always used whatever _compute_free_signals() auto-ranked top today, which on a quiet market can repeat the same handful of tickers for days): tickers is an optional comma-separated list (e.g. "NVDA,BTC-USD,GC=F"). Blank (the default, and what the unattended daily schedule still passes) keeps the original auto-picked today's-top-signals behavior unchanged -- see services/video_engine_service.py's generate_daily_video()/ _build_custom_signals() docstrings.