feat: serve responsive WebP srcsets for faster mobile browsing

On-demand ?w= resize with a disk cache under webcache/; downloads stay full-resolution.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/main.py

@@ -13,7 +13,7 @@ from fastapi.responses import FileResponse, HTMLResponse, RedirectResponse
 from fastapi.staticfiles import StaticFiles
 from fastapi.templating import Jinja2Templates
 
-from . import config, pipeline, remix
+from . import config, images, pipeline, remix
 from .logging_config import configure_logging
 
 configure_logging()
@@ -23,6 +23,9 @@ app = FastAPI(title=config.SITE_TITLE)
 
 BASE_DIR = Path(__file__).resolve().parent
 templates = Jinja2Templates(directory=str(BASE_DIR / "templates"))
+templates.env.globals["img_attrs"] = images.img_attrs
+templates.env.globals["PAIR_SIZES"] = images.PAIR_SIZES
+templates.env.globals["GRID_SIZES"] = images.GRID_SIZES
 app.mount("/static", StaticFiles(directory=str(BASE_DIR / "static")), name="static")
 
 # Job id = sanitized source stem (may include dots).
@@ -120,9 +123,25 @@ def job_detail(request: Request, job_id: str) -> HTMLResponse:
 
 
 @app.get("/jobs/{job_id}/files/{filename:path}")
-def job_file(job_id: str, filename: str) -> FileResponse:
+def job_file(
+    job_id: str,
+    filename: str,
+    w: int | None = Query(None, description="Longest-edge width for srcset WebP"),
+) -> FileResponse:
     path = _safe_job_file(job_id, filename)
-    return FileResponse(path)
+    if w is None:
+        return FileResponse(path)
+    if w not in images.SRCSET_WIDTHS:
+        raise HTTPException(
+            status_code=400,
+            detail=f"Ungueltige Breite (erlaubt: {', '.join(map(str, images.SRCSET_WIDTHS))})",
+        )
+    try:
+        cached = images.get_or_create_resized(path, w)
+    except OSError as exc:
+        logger.warning("resize failed for %s w=%s: %s", path.name, w, exc)
+        raise HTTPException(status_code=500, detail="Bild konnte nicht skaliert werden") from exc
+    return FileResponse(cached, media_type="image/webp", filename=cached.name)
 
 
 @app.get("/jobs/{job_id}/remix", response_class=HTMLResponse)