Add nested posts UI (#5)

## Summary
- Cut hunt, question, submission, voting, hiding, and profile flows over to unified posts
- Render nested replies with permission-aware inline Reply/Edit controls and edited markers
- Add post profile queries and the author index migration they depend on

Co-authored-by: codegirl-007 <s.raide@gmail.com>

templates/partials/_post.html

@@ -0,0 +1,67 @@
+{{define "postActions"}}
+<div class="post-actions">
+  {{if canReply .User .Root}}
+    <details class="post-composer">
+      <summary>Reply</summary>
+      <form class="post-form" method="post" action="/posts">
+        <input type="hidden" name="_csrf" value="{{.CSRF}}">
+        <input type="hidden" name="parent_id" value="{{.Post.ID}}">
+        <label for="reply-{{.Post.ID}}">Reply to {{.Post.AuthorName}}</label>
+        <textarea id="reply-{{.Post.ID}}" name="body" rows="5" required maxlength="12000"></textarea>
+        <div class="post-form-actions">
+          <button type="submit" class="btn btn-primary">Post reply</button>
+          <button type="reset" class="btn btn-ghost"
+            onclick="this.closest('details').removeAttribute('open')">Cancel</button>
+        </div>
+      </form>
+    </details>
+  {{end}}
+  {{if canEditPost .User .Post}}
+    <details class="post-composer">
+      <summary>Edit</summary>
+      <form class="post-form" method="post" action="/posts/{{.Post.ID}}/edit">
+        <input type="hidden" name="_csrf" value="{{.CSRF}}">
+        <label for="edit-{{.Post.ID}}">Edit post</label>
+        <textarea id="edit-{{.Post.ID}}" name="body" rows="5" required
+          maxlength="12000">{{.Post.Body}}</textarea>
+        <div class="post-form-actions">
+          <button type="submit" class="btn btn-primary">Save changes</button>
+          <button type="reset" class="btn btn-ghost"
+            onclick="this.closest('details').removeAttribute('open')">Cancel</button>
+        </div>
+      </form>
+    </details>
+  {{end}}
+  {{if and (not .Post.ParentID) (isAdmin .User)}}
+    <form class="post-hide" method="post" action="/questions/{{.Post.ID}}/hide">
+      <input type="hidden" name="_csrf" value="{{.CSRF}}">
+      <button type="submit" class="linkish">Hide</button>
+    </form>
+  {{end}}
+</div>
+{{end}}
+
+{{define "threadReply"}}
+{{$ctx := .}}
+<article id="post-{{.Post.ID}}"
+  class="thread-post thread-post-{{postDepth .Depth}}{{if eq .Post.AuthorRole "admin"}} is-shop{{end}}">
+  <header class="post-head">
+    <p class="post-kicker">{{postLabel .Post}}</p>
+    <p class="post-meta">
+      <span>{{.Post.AuthorName}}</span>
+      <span class="dot" aria-hidden="true">·</span>
+      <time datetime="{{.Post.CreatedAt}}">{{postTime .Post.CreatedAt}}</time>
+      {{if isEdited .Post}}<span class="edited">Edited</span>{{end}}
+    </p>
+  </header>
+  <p class="post-body">{{.Post.Body}}</p>
+  {{template "postActions" .}}
+  {{if .Post.Replies}}
+    <div class="post-replies">
+      {{range .Post.Replies}}
+        {{template "threadReply" (postCtx $ctx.User $ctx.CSRF $ctx.Root . (add $ctx.Depth 1))}}
+      {{end}}
+    </div>
+  {{end}}
+</article>
+{{end}}