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

pyproject.toml

@@ -40,6 +40,7 @@ serve = [
     "uvicorn[standard]>=0.29",
     "python-multipart>=0.0.9",
     "bcrypt>=4.1",
+    "Pillow>=10.0",
 ]
 strava = [
     "requests>=2.32",