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.

README.md

@@ -185,6 +185,7 @@ Perfect for remote print farms, traveling makers, or accessing your home printer
 - Track plates (print jobs) and parts separately
 - Auto-detect parts count from 3MF files
 - Color-coded project badges
+- **Project URL + cover photo** — paste a MakerWorld/Printables/Thingiverse link and upload a hero image so each card is immediately recognisable; the URL renders as a one-click link beside the project name
 - Bulk assign archives via multi-select toolbar
 - Import/Export projects as ZIP (includes files) or JSON
 - Print or queue files from linked library folders directly in the project view (resulting archive auto-linked to the project)