feat(projects): URL field + cover photo on project cards (#1155)

Two new project fields: a free-text URL rendered as a one-click
  external-link button beside the project name on every card (opens in a
  new tab, click is e.stopPropagation()-guarded so it doesn't enter the
  project), and a cover photo that replaces the status-icon box with a
  square thumbnail.

  URL is plumbed through ProjectCreate/Update/Response/ListResponse,
  including from-template + create-template flows so it inherits between
  a project and its template. Cover photo is not inherited because the
  file would be shared on disk between source and copy.

  Schema validator rejects anything other than http:// or https://
  prefixes -- <a href> rendering would otherwise execute javascript:
  / data: / file: URLs even with React's default escaping. PATCH uses
  model_fields_set for the URL field so users can clear it by sending
  {"url": null}.

  Cover image storage: Project.cover_image_filename references a file
  Cover image storage: Project.cover_image_filename references a file
  inside the existing archives/projects/{id}/attachments/ dir, but it's
  tracked separately from the attachments JSON list so swap/delete on
  the cover doesn't perturb the user's other attachments. Three routes
  (POST/GET/DELETE /projects/{id}/cover-image) accept only .jpg/.jpeg/
  .png/.gif/.webp (no SVG -- SVG can carry script payloads), replace in
  place (prior file deleted before the new one lands so repeat uploads
  can't accumulate orphans), and self-heal when a DB reference points at
  a vanished disk file by clearing the column and 404'ing.

  GET cover-image is gated by RequireCameraStreamTokenIfAuthEnabled
  (accepts ?token=... query string) -- not the bearer-token gate -- so
  <img src> requests work in both auth-on and auth-off configurations.
  The frontend wraps getProjectCoverImageUrl with withStreamToken(),
  matching the existing pattern from getArchiveThumbnail.

  Permissions: PROJECTS_UPDATE for upload/delete/PATCH, PROJECTS_READ
  gate is implicit via the stream-token credential. Migration: 2
  idempotent ALTER TABLE projects ADD COLUMN. Localised across all 8
  UI languages.

backend/tests/unit/test_printer_manager_status_broadcast.py

@@ -32,6 +32,23 @@ def manager():
     return PrinterManager()
 
 
+def _close_unawaited(coro):
+    """Side effect for mocked ``_schedule_async``.
+
+    ``set_awaiting_plate_clear`` evaluates the coroutine expressions
+    ``self._persist_awaiting_plate_clear(...)`` and
+    ``self._broadcast_status_change(...)`` before passing them to
+    ``_schedule_async``. When that target is patched, the coroutine objects
+    leak — Python's ``__del__`` then emits ``coroutine was never awaited``
+    during GC, and when GC runs late enough that warning hits the interpreter
+    shutdown path with ``KeyError: '__import__'``. Closing the coroutine here
+    prevents both. Returns ``None`` so the mock's call signature is unchanged.
+    """
+    if asyncio.iscoroutine(coro):
+        coro.close()
+    return None
+
+
 def _fake_state(**overrides):
     """Minimal stand-in for a ``PrinterState`` — only the attributes
     ``printer_state_to_dict`` reads. We use a SimpleNamespace rather than
@@ -58,7 +75,7 @@ class TestSchedulingFromSetAwaitingPlateClear:
         manager._loop = MagicMock()
         manager._loop.is_running.return_value = True
 
-        with patch.object(manager, "_schedule_async") as scheduled:
+        with patch.object(manager, "_schedule_async", side_effect=_close_unawaited) as scheduled:
             manager.set_awaiting_plate_clear(7, True)
 
         # Two coroutines: persist + broadcast. Order doesn't matter.
@@ -96,7 +113,7 @@ class TestSchedulingFromSetAwaitingPlateClear:
         manager._loop = MagicMock()
         manager._loop.is_running.return_value = True
 
-        with patch.object(manager, "_schedule_async") as scheduled:
+        with patch.object(manager, "_schedule_async", side_effect=_close_unawaited) as scheduled:
             manager.set_awaiting_plate_clear(7, True)
             scheduled.reset_mock()
             manager.set_awaiting_plate_clear(7, False)