Add an image field, required for WordPress posts

Posts get a nullable image_url. WordPressPublisher now downloads it
and uploads it to WordPress's media endpoint before creating the
post, setting it as featured_media — and rejects publishing (without
touching status) if a blog post has no image set, mirroring the
existing title requirement. Every other network treats it as optional.

Not enforced at save time, same as title — you can still draft an
incomplete WordPress post, it just can't be published until an image
is set. Exposed through the UI form (with a thumbnail/""missing""
indicator in the table), the JSON API, and the Python CLI wrapper.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

AI_API.md

@@ -37,6 +37,8 @@ SocialAccount  (login for a social network)
 Post  (one piece of content, belongs to a Site, optionally a SocialAccount)
   └─ type: same vocabulary as SocialAccount.network, plus "blog"
   └─ target: nullable, network-specific destination (e.g. a subreddit for type=reddit)
+  └─ image_url: nullable URL. Required to publish type=blog (WordPress) —
+     used as the featured image. Optional for every other type.
   └─ status: draft -> scheduled -> publishing -> published | failed
   └─ published_url: set once it's actually live somewhere
   └─ publish_error: set to the failure reason when status=failed (or when a
@@ -145,6 +147,7 @@ Body:
   "title": "Optional title",
   "body": "Required content",
   "target": null,
+  "image_url": null,
   "status": "draft",
   "scheduled_at": null
 }
@@ -153,8 +156,10 @@ Validation: `site_id` required, must exist. `social_account_id` nullable,
 must exist if given. `type` required, one of `twitter`, `instagram`,
 `facebook`, `reddit`, `tiktok`, `linkedin`, `youtube`, `blog`, `other`.
 `body` required. `title` nullable. `target` nullable — a network-specific
-destination, e.g. the subreddit name for `type=reddit`. `status` defaults to
-`draft` if omitted. `scheduled_at` nullable datetime.
+destination, e.g. the subreddit name for `type=reddit`. `image_url` nullable
+URL — not enforced at save time (you can draft a `type=blog` post without one),
+but `WordPressPublisher` will reject publishing it until it's set. `status`
+defaults to `draft` if omitted. `scheduled_at` nullable datetime.
 
 Note: `type` and the linked Social Account's `network` are **not**
 cross-validated — you can link a `type=blog` post to a Twitter account, or
@@ -192,7 +197,7 @@ who/what is driving:
 - **Manually**, for anything without a wired-in publisher yet: do it
   yourself and `PATCH` the result back (see below).
 
-Note the asymmetry: WordPress is synchronous (a single HTTP call, fast
+Note the asymmetry: WordPress is synchronous (a few plain HTTP calls, fast
 enough to run inline), Reddit is not (it's real browser automation via
 Playwright, tens of seconds) — which is exactly why the UI path is
 queue-backed rather than a direct synchronous call.
@@ -212,17 +217,20 @@ php artisan posts:publish {post_id}
 What it does:
 1. Loads the Post and its Site.
 2. Validates: `post.type` must be `blog`, `post.title` must be set,
-   `site.cms_type` must be `wordpress`, and the site must have
-   `wp_username`/`wp_app_password` set. Any failure here exits 1 with a
-   descriptive message and **does not touch** `Post.status`.
-3. POSTs to `{site.url}/wp-json/wp/v2/posts` using HTTP Basic Auth with
-   the site's WordPress Application Password, `status: publish` (goes live
-   immediately, not a WP draft).
-4. On success: sets `Post.status = published`, `Post.published_url` to the
+   `post.image_url` must be set, `site.cms_type` must be `wordpress`, and
+   the site must have `wp_username`/`wp_app_password` set. Any failure here
+   exits 1 with a descriptive message and **does not touch** `Post.status`.
+3. Downloads `post.image_url`, then POSTs it to
+   `{site.url}/wp-json/wp/v2/media` (HTTP Basic Auth with the site's
+   WordPress Application Password) to upload it as media and get back a
+   media id.
+4. POSTs to `{site.url}/wp-json/wp/v2/posts` with `status: publish` (goes
+   live immediately, not a WP draft) and `featured_media` set to that id.
+5. On success: sets `Post.status = published`, `Post.published_url` to the
    new post's permalink, prints `{"success": true, "url": "..."}`, exits 0.
-5. On failure (bad credentials, unreachable site, WordPress error): sets
-   `Post.status = failed`, prints `{"success": false, "error": "..."}`,
-   exits 1.
+6. On failure at either HTTP step (bad credentials, unreachable site,
+   unreachable/rejected image, WordPress error): sets `Post.status = failed`,
+   prints `{"success": false, "error": "..."}`, exits 1.
 
 This is a **shell command**, not an HTTP endpoint — run it from wherever
 you have shell access to this app (e.g. `php artisan posts:publish 42`).