fix: address PR #57 CodeRabbit feedback

- Acquire AniList post-watch in-flight lock before async gating to prevent duplicate writes
- Isolate manual watched mark result from AniList post-watch callback failures
- Report known-word cache clears as mutations during immediate append when state existed
- Add regression tests for each fix

changes/334-coderabbit-followups.md

@@ -0,0 +1,4 @@
+type: fixed
+area: anilist
+
+- AniList: Prevented duplicate post-watch writes during concurrent checks, preserved manual watched marks when post-watch sync fails, and kept known-word cache refresh notifications accurate after cache resets.