Fix OpenAPI docs, add image import, file preview modal, keyboard nav

- Fix OpenAPI YAML: correct components/schemas indentation (was inside paths:)
- Downgrade openapi version 3.1.0 → 3.0.3 for ReDoc 2.5.2 compatibility
- Add /import/images/<path> endpoint with worker-mode background download
- Add source_url field to FileUpload model (migration 0044)
- Add file preview modal with image/video/audio/unsupported cases
- Add arrow key navigation + prev/next buttons + file counter in modal
- Add /import/images/{image_url} to OpenAPI spec with source_url field

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

static/openapi.yaml

@@ -1,4 +1,4 @@
-openapi: 3.1.0
+openapi: 3.0.3
 info:
   title: GoLinks API
   description: API documentation for GoLinks service
@@ -396,6 +396,56 @@ paths:
           description: File content (inline for images, attachment for others)
         '404':
           description: File not found, is private, or link has expired
+
+  /import/images/{image_url}:
+    get:
+      summary: Import and cache an external image
+      description: |
+        Imports an external image by URL and saves it to the local file store.
+
+        The `image_url` path parameter is the image URL **without** the `https://` scheme prefix
+        (e.g. `example.com/path/to/image.jpg`). The server always tries `https://` first.
+
+        **Worker mode** — this endpoint is non-blocking:
+        - On the **first request** for a given URL the server creates a `FileUpload` record
+          (marked `is_public: true`) and kicks off a background thread to download and save the file.
+          The response immediately redirects (HTTP 302) to the original `https://` source URL so the
+          image is visible right away.
+        - On **subsequent requests**, once the background download has completed, the response
+          redirects to the locally-saved public file URL (`/public/files/{uuid}-{filename}`) so the
+          original host is no longer needed.
+
+        **Idempotent** — the same external URL always resolves to the same `FileUpload` record;
+        the file is only downloaded once.
+
+        **Use in Markdown / posts:**
+        ```
+        ![Alt text](http://your-domain/import/images/example.com/path/to/image.jpg)
+        ```
+      tags:
+        - Files
+      parameters:
+        - in: path
+          name: image_url
+          required: true
+          schema:
+            type: string
+          description: |
+            External image URL without the scheme prefix.
+            Example: `graziamagazine.com/wp-content/uploads/2024/12/Elle-Fanning-Pigtails-scaled.jpg`
+          example: graziamagazine.com/wp-content/uploads/2024/12/Elle-Fanning-Pigtails-scaled.jpg
+      responses:
+        '302':
+          description: |
+            Redirect to either the original source URL (while background download is in progress)
+            or the locally-saved public file URL (once download has completed).
+          headers:
+            Location:
+              schema:
+                type: string
+              description: URL to redirect to (original source or local `/public/files/…`)
+
+components:
   schemas:
     Page:
       type: object
@@ -633,6 +683,14 @@ paths:
         download_count:
           type: integer
           readOnly: true
+        source_url:
+          type: string
+          format: uri
+          nullable: true
+          description: |
+            Original external URL this file was imported from via `/import/images/…`.
+            `null` for files uploaded directly.
+          example: https://example.com/path/to/image.jpg
         created_at:
           type: string
           format: date-time