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/static/js/VoteManager.js

@@ -1,4 +1,5 @@
 import { Toast } from "./Toast.js";
+import { Http } from "./Http.js";
 
 export class VoteManager {
     constructor() {
@@ -20,18 +21,7 @@ export class VoteManager {
         const action = form.getAttribute("action");
         const value = form.querySelector('input[name="value"]').value;
         try {
-            const response = await fetch(action, {
-                method: "POST",
-                headers: {
-                    "X-Requested-With": "fetch",
-                    "Content-Type": "application/x-www-form-urlencoded",
-                },
-                body: new URLSearchParams({ value }),
-            });
-            if (!response.ok) {
-                throw new Error(`vote failed with status ${response.status}`);
-            }
-            const result = await response.json();
+            const result = await Http.sendForm(action, { value });
             this.render(action, result);
         } catch (error) {
             console.error("vote failed", error);