## Implementation Plan: Notify Previous Commenters on New Comments

### 1. Add New Notification Type

**File:** `devplacepy/database/notifications.py`

- Add `"thread_comment"` to the `NOTIFICATION_TYPES` tuple (or list).  
- Insert it in alphabetical order (or append at the end) after `"system"`. No message template needed here; the message will be constructed at call site.

### 2. Modify `create_comment_record()` in `devplacepy/content.py`

**Location:** After existing notification logic (line 361), before the audit event emission (line 373).  
**Scope:** This change must apply to both top-level comments and replies, but only for targets of type `"post"`.

**Steps:**

1. After the existing `if target_type == "post":` block, add a new inner block that fires only when the comment is not a reply (`parent_uid is None`) or, for consistency, after the reply notification has been sent. A simpler approach: place the new logic **after** the existing `if-else` block, guarded by `if target_type == "post":`.

2. Query the `comments` table for distinct `user_uid` values where:
   - `target_type = 'post'`
   - `target_uid = post_uid` (the post UID, already available as `target_uid`)
   - `deleted_at = None`
   - **Exclude** the current comment author (`user["uid"]`)
   - **Exclude** the post owner (already notified via `"comment"` type)
   - **Exclude** the parent comment author, if this is a reply (already notified via `"reply"` type)

   Use a single SQL query with `DISTINCT` or fetch all rows and deduplicate in Python.

3. For each qualifying `user_uid`, call:
   ```python
   create_notification(
       user_uid,
       "thread_comment",
       f"{user['username']} also commented on a post you commented on",
       user["uid"],
       comment_url,
   )
   ```

4. Ensure no duplicate notifications are created if a user qualifies for both a reply and this new notification (already handled by exclusions).

**Edge cases:**
- Post has no previous commenters → no new notifications sent.
- Previous commenter is also the post author → already excluded.
- Comment author is also a previous commenter → excluded.

### 3. Update E2E Test Coverage

**File:** `tests/e2e/notifications/index.py`

Add a new test function, e.g. `test_thread_comment_notification`, that:

1. Registers three users: `post_owner`, `first_commenter`, `second_commenter`.
2. `post_owner` creates a post.
3. `first_commenter` adds a top-level comment on that post.
4. `second_commenter` adds another top-level comment on the same post.
5. Asserts that `first_commenter` receives exactly one notification of type `"thread_comment"` (message should contain `"also commented"`).
6. Asserts that `post_owner` receives exactly one notification of type `"comment"` (unchanged).
7. Optionally verify that `second_commenter` receives no self-notification.

Place this test after existing comment notification tests (around line 381). Use the same Playwright fixtures and helpers as the surrounding tests to avoid introducing new dependencies.

### 4. Run Verification

Execute the project’s test command:
```bash
pip install -e '.[dev]' -q && python -m pytest tests/e2e/notifications/index.py -x
```
Initially target the single file to validate the new test. Then run the full suite:
```bash
pip install -e '.[dev]' -q && python -m pytest
```
to confirm no regressions.

---

## Definition of Done

- [ ] `"thread_comment"` is added to `NOTIFICATION_TYPES` in `devplacepy/database/notifications.py`.
- [ ] In `devplacepy/content.py`, `create_comment_record()` contains new logic that queries the `comments` table for prior commenters on the same post (excluding the comment author, post owner, and parent commenter if replying) and calls `create_notification()` for each with type `"thread_comment"`.
- [ ] A new e2e test `test_thread_comment_notification` exists in `tests/e2e/notifications/index.py` that verifies a previous commenter receives a notification when another user comments on the same post.
- [ ] The project test command (`pip install -e '.[dev]' -q && python -m pytest`) passes with zero errors.
- [ ] All existing e2e tests for comments and notifications (including Playwright-based tests) continue to pass with no new failures.