feat(15-02): add Posts admin with mlmarkdown and FormatHTML

YAML-driven Posts screen stores translatable markdown and regenerates
content_html in the plugin. Publish writes without access_publish are
refused; list and form queries stay on the principal without
access_other_posts.

Co-authored-by: Cursor <cursoragent@cursor.com>

models/post/columns.yaml

@@ -0,0 +1,25 @@
+columns:
+    title:
+        label: golem15.journal::lang.post.title
+        searchable: true
+    is_pinned:
+        label: Pinned
+        type: switch
+        sortable: true
+    categories:
+        label: golem15.journal::lang.post.categories
+        relation: categories
+        select: name
+        searchable: true
+        sortable: false
+    created_at:
+        label: golem15.journal::lang.post.created
+        type: date
+        invisible: true
+    updated_at:
+        label: golem15.journal::lang.post.updated
+        type: date
+        invisible: true
+    published_at:
+        label: golem15.journal::lang.post.published
+        type: date

models/post/fields.yaml

@@ -0,0 +1,67 @@
+fields:
+    title:
+        label: golem15.journal::lang.post.title
+        type: mltext
+        span: left
+        tab: golem15.journal::lang.post.tab_edit
+        required: true
+    slug:
+        label: golem15.journal::lang.post.slug
+        type: mltext
+        span: right
+        tab: golem15.journal::lang.post.tab_edit
+        required: true
+    content:
+        label: golem15.journal::lang.post.content
+        type: mlmarkdown
+        size: huge
+        tab: golem15.journal::lang.post.tab_edit
+        required: true
+    excerpt:
+        label: golem15.journal::lang.post.excerpt
+        type: mltext
+        size: small
+        tab: golem15.journal::lang.post.tab_manage
+    categories:
+        label: golem15.journal::lang.post.categories
+        type: relation
+        nameFrom: name
+        tab: golem15.journal::lang.post.tab_categories
+        comment: golem15.journal::lang.post.categories_comment
+    tags:
+        label: golem15.journal::lang.journal.tags
+        type: relation
+        nameFrom: name
+        tab: golem15.journal::lang.post.tab_categories
+        comment: golem15.journal::lang.post.tags_comment
+    published:
+        label: golem15.journal::lang.post.published
+        type: switch
+        span: left
+        tab: golem15.journal::lang.post.tab_manage
+    is_pinned:
+        label: Pinned
+        comment: Pin this post to appear first in listings
+        type: checkbox
+        span: left
+        tab: golem15.journal::lang.post.tab_manage
+    user:
+        label: golem15.journal::lang.post.published_by
+        type: relation
+        nameFrom: login
+        emptyOption: golem15.journal::lang.post.current_user
+        span: right
+        tab: golem15.journal::lang.post.tab_manage
+    published_at:
+        label: golem15.journal::lang.post.published_on
+        type: datepicker
+        mode: datetime
+        span: left
+        tab: golem15.journal::lang.post.tab_manage
+    featured_images:
+        label: golem15.journal::lang.post.featured_images
+        type: fileupload
+        mode: image
+        imageWidth: 200
+        imageHeight: 200
+        tab: golem15.journal::lang.post.tab_manage