feat: OG link previews — track image + meta tags for Telegram/WhatsApp

- bincio/render/ogimage.py: generate 400x400 elevation-coloured PNG with Pillow
- bincio/serve/routers/ogimage.py: /activity/{id}/ OG HTML stub for bot UAs;
  /og-image/{user}/{id}.png serves pre-generated images with on-demand fallback
- scripts/generate_og_images.py: batch pre-generation, incremental (mtime skip)
- scripts/strava_elevation_audit.py: add source/threshold/MA columns and pct stats
- pyproject.toml: add Pillow>=10 to serve extras

bincio/serve/server.py

@@ -25,6 +25,7 @@ from bincio.serve.routers import (
     garmin,
     ideas,
     me,
+    ogimage,
     segments,
     strava,
     uploads,
@@ -67,5 +68,6 @@ for _router in [
     strava.router,
     garmin.router,
     ideas.router,
+    ogimage.router,
 ]:
     app.include_router(_router)