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/_comment_section.html

@@ -6,12 +6,12 @@
         <div class="comment-votes">
             <form method="POST" action="/votes/comment/{{ item.comment['uid'] }}">
                 <input type="hidden" name="value" value="1">
-                <button type="submit" class="comment-vote-btn">+</button>
+                <button type="submit" class="comment-vote-btn vote-up{% if item.my_vote == 1 %} voted{% endif %}">+</button>
             </form>
             <span class="comment-vote-count" data-vote-count="{{ item.comment['uid'] }}">{{ item.votes.up - item.votes.down }}</span>
             <form method="POST" action="/votes/comment/{{ item.comment['uid'] }}">
                 <input type="hidden" name="value" value="-1">
-                <button type="submit" class="comment-vote-btn">-</button>
+                <button type="submit" class="comment-vote-btn vote-down{% if item.my_vote == -1 %} voted{% endif %}">-</button>
             </form>
         </div>