feat: floor plan viewport rendering, per-canvas config, server media upload

- Render floor plan inside React Flow ViewportPortal so it pans/zooms with
  nodes (was screen-fixed, desynced on pan/zoom); zoom-stable resize handles.
- Move floor plan config from the left panel into the canvas (design) edit
  modal; attach per-design and fix cross-design bleed on load.
- Store images via a new generic backend media endpoint (POST/GET/DELETE
  /api/v1/media) on disk under <data_dir>/uploads, not base64 in the canvas.
- Disable floor plans in standalone mode (no backend to upload/serve); drop
  base64 localStorage persistence. See ADR-001 in CLAUDE.md.
- Tests: backend media route, DesignModal floor plan + upload, store floorMap.

ha-relevant: maybe

backend/app/core/config.py

@@ -24,6 +24,10 @@ class Settings(BaseSettings):
 
     secret_key: str  # Required — set SECRET_KEY in .env
     sqlite_path: str = "./data/homelab.db"
+    # Uploaded media (floor plans, and future raw image uploads) live on disk,
+    # not in the DB. Defaults to a `uploads/` folder next to the SQLite DB so it
+    # sits on the same persistent Docker volume. Override with UPLOAD_DIR.
+    upload_dir: str = ""
     cors_origins: list[str] = ["http://localhost:5173", "http://localhost:3000"]
 
     # JWT
@@ -79,6 +83,13 @@ class Settings(BaseSettings):
     def _override_path(self) -> Path:
         return Path(self.sqlite_path).parent / "scan_config.json"
 
+    def media_dir(self) -> Path:
+        """On-disk folder for uploaded media. Sits on the same persistent
+        volume as the SQLite DB unless UPLOAD_DIR is set."""
+        if self.upload_dir:
+            return Path(self.upload_dir)
+        return Path(self.sqlite_path).parent / "uploads"
+
     def load_overrides(self) -> None:
         """Load runtime scan config overrides written by the API."""
         try: