feat: add leaderboard router, gamification backfill, and shared content creation helpers

Introduce a new `/leaderboard` endpoint ranking top 50 members by total stars, wire it into the app router and documentation. Implement `_backfill_gamification()` in `database.py` to compute XP/levels for existing accounts from prior posts, comments, votes, and follows. Extract `is_owner()`, `create_content_item()`, and `detail_context()` into `content.py` to centralize content creation, reward awarding, mention notifications, and attachment linking. Update comment creation/deletion in `comments.py` to use `award_rewards()` with `XP_COMMENT` and the new `is_owner()` helper. Add shared template partials documentation for vote bars, star buttons, post headers, and topic selectors to `AGENTS.md`.

devplacepy/templates/_post_header.html

@@ -0,0 +1,10 @@
+<div class="post-header">
+    {% set _user = _author %}{% set _size = 32 %}{% set _size_class = "sm" %}{% include "_avatar_link.html" %}
+    <div class="post-author-wrap">
+        <a href="/profile/{{ _author['username'] if _author else '#' }}" class="post-author-link">{{ _author['username'] if _author else 'Unknown' }}</a>
+        {% if _author and _author.get('role') %}
+        <span class="post-author-role">{{ _author['role'] }}</span>
+        {% endif %}
+    </div>
+    <span class="post-time">{{ _time }}</span>
+</div>