feat: edit existing manga metadata (0.31.0)

Adds PUT /mangas/:id/cover (multipart) and DELETE /mangas/:id/cover so
covers can be replaced or cleared after creation, and wires a dedicated
/manga/[id]/edit SvelteKit route that combines the existing PATCH with
the new cover endpoints. Cover PUT cleans up the old blob when the
extension changes, swallowing StorageError::NotFound so a manually-gone
file doesn't surface as a 404 to the client. Edit link on the manga
detail page is gated on session.user, matching the auth posture of the
underlying handlers.

Also pins the local-dev port story via loadEnv() in vite.config.ts so
VITE_PORT / BACKEND_URL from a (gitignored) .env keep the dev URL
stable across runs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/repo/manga.rs

@@ -262,6 +262,17 @@ pub async fn set_cover_image_path<'e, E: PgExecutor<'e>>(
     Ok(())
 }
 
+pub async fn clear_cover_image_path<'e, E: PgExecutor<'e>>(
+    executor: E,
+    id: Uuid,
+) -> AppResult<()> {
+    sqlx::query("UPDATE mangas SET cover_image_path = NULL, updated_at = now() WHERE id = $1")
+        .bind(id)
+        .execute(executor)
+        .await?;
+    Ok(())
+}
+
 pub async fn exists(pool: &PgPool, id: Uuid) -> AppResult<bool> {
     let (exists,): (bool,) =
         sqlx::query_as("SELECT EXISTS(SELECT 1 FROM mangas WHERE id = $1)")