fix: test DB isolation, Discord recovery, and over-mocked tests

Test data was bleeding into production tasks.db because
swarm.task_queue.models.DB_PATH (relative path) was never patched in
conftest.clean_database. Fixed by switching to absolute paths via
settings.repo_root and adding the missing module to the patching list.

Discord bot could leak orphaned clients on retry after ERROR state.
Added _cleanup_stale() to close stale client/task before each start()
attempt, with improved logging in the token watcher.

Rewrote test_paperclip_client.py to use httpx.MockTransport instead of
patching _get/_post/_delete — tests now exercise real HTTP status codes,
error handling, and JSON parsing. Added end-to-end test for
capture_error → create_task DB isolation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/dashboard/app.py

@@ -235,10 +235,18 @@ async def _discord_token_watcher() -> None:
 
         if token:
             try:
+                logger.info(
+                    "Discord watcher: token found, attempting start (state=%s)",
+                    discord_bot.state.name,
+                )
                 success = await discord_bot.start(token=token)
                 if success:
                     logger.info("Discord bot auto-started (token detected)")
                     return  # Done — stop watching
+                logger.warning(
+                    "Discord watcher: start() returned False (state=%s)",
+                    discord_bot.state.name,
+                )
             except Exception as exc:
                 logger.warning("Discord auto-start failed: %s", exc)