feat(nadia): Implement participation notification in comment creation

**Outcome:** done
**Changed:** devplacepy/content.py:363-384
**Verified by:** `python3 -m py_compile devplacepy/content.py` passed, `python3 -m pyflakes devplacepy/content.py` clean
**Findings:**
- `devplacepy/content.py:363-384` — participation notification block inserted inside `if target_type == "post"`, after existing reply/comment notifications, before `create_mention_notifications`
- Logic: fetches post owner, queries distinct previous commenters, excludes self and post owner, calls `create_notification` for each
- Silencing handled automatically by `_deliver_notification` in `devplacepy/utils/notifications.py:103`
- `NOTIFICATION_TYPES` at `devplacepy/database/notifications.py:15` already contains `"participation"` key
- No new imports required; query uses existing `idx_comments_target` index
**Open:** Full `make test` regression pass needs Python >=3.12 environment
**Confidence:** high — compile and pyflakes both pass; minimal bounded addition following existing notification pattern; all preconditions verified before editing

Typosaurus-Run: 33c1c53eb8d34b528ee5beab574476fb
Typosaurus-Node: 67dfba8421ba47ea863bc2b87eb01cdd
Typosaurus-Agent: @nadia
Refs: #132

devplacepy/content.py

@@ -360,6 +360,29 @@ def create_comment_record(
                     comment_url,
                 )
 
+        # Notify previous commenters on this post (participation)
+        posts = get_table("posts")
+        post = posts.find_one(uid=target_uid)
+        if not post:
+            post = posts.find_one(slug=target_uid)
+        if post:
+            post_owner_uid = post["user_uid"]
+            previous_commenters = set()
+            for c in get_table("comments").find(
+                target_type="post", target_uid=target_uid, deleted_at=None
+            ):
+                cu = c["user_uid"]
+                if cu != user["uid"] and cu != post_owner_uid:
+                    previous_commenters.add(cu)
+            for cu in previous_commenters:
+                create_notification(
+                    cu,
+                    "participation",
+                    f"{user['username']} also commented on this post",
+                    user["uid"],
+                    comment_url,
+                )
+
     create_mention_notifications(content, user["uid"], comment_url)
     schedule_correction(user, "comments", comment_uid, request)
     schedule_modification(user, "comments", comment_uid, request)
@@ -718,3 +741,4 @@ def enrich_items(
             )
         enriched.append(entry)
     return enriched
+