Feat/anonymous replies fix clean

[JoshuaNam]

Changes (Issue #29 + #6)

• Added backend support for marking replies/posts as anonymous
• Persisted anonymous flag (0/1) on posts in the database
• Masked poster identity for non-admin users (uid set to 0, username/display name set to "Anonymous", generic avatar)
• Preserved real author visibility for admins and exposed isAnonymous=true flag
• Ensured non-anonymous posts are unaffected
• Added unit tests covering storage, API masking, admin visibility, topic-level masking, and regression cases

Testing

1. Run the full backend test suite:
   npm test

2. Verify:

• Anonymous posts store anonymous=1
• Regular users see masked identity (uid=0, username "Anonymous")
• Admins see real author with isAnonymous=true
• Non-anonymous posts behave normally

3. Confirm all existing tests pass with no regressions

Related Issues

Resolves #6
Resolves #29