docs(backlog): Add code TODOs from codebase to PRD_BACKLOG
Added infrastructure/technical TODOs:
- OAuth authentication for community actions (P1)
- Jetstream consumer race condition (P2)
- Structured logging migration (P3)
- PDS URL resolution from DID (P3)
- PLC directory registration for prod (P3)
Feature-specific TODOs (avatars, moderator checks, update/delete handlers)
are tracked in their respective PRDs (PRD_COMMUNITIES, PRD_GOVERNANCE).
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Co-Authored-By: Claude <noreply@anthropic.com>