refactor(backend): Integrate autogpt_libs into backend structure (OPEN-2998)

Properly integrates autogpt_libs modules into the backend's existing
structure instead of just moving them wholesale.

Structure changes:
- auth/ → backend/api/auth/ (FastAPI auth dependencies)
- api_key/ → backend/api/auth/api_key/ (API key auth)
- logging/ → backend/logging/ (structured logging config)
- utils/synchronize → backend/util/synchronize.py

Removed (unused):
- rate_limit/ - backend has its own rate limiting
- supabase_integration_credentials_store/ - not imported anywhere

Import path changes:
- autogpt_libs.auth.* → backend.api.auth.*
- autogpt_libs.api_key.* → backend.api.auth.api_key.*
- autogpt_libs.logging.* → backend.logging.*
- autogpt_libs.utils.synchronize → backend.util.synchronize

Also updates:
- pyproject.toml (merged deps, removed path ref)
- Dockerfile (removed autogpt_libs copy)
- CI workflow (removed autogpt_libs paths)
- dependabot.yml (removed autogpt_libs entry)
- Docs (CLAUDE.md, TESTING.md)

Ticket: https://linear.app/autogpt/issue/OPEN-2998

.github/workflows/platform-backend-ci.yml

@@ -6,13 +6,11 @@ on:
     paths:
       - ".github/workflows/platform-backend-ci.yml"
       - "autogpt_platform/backend/**"
-      - "autogpt_platform/autogpt_libs/**"
   pull_request:
     branches: [master, dev, release-*]
     paths:
       - ".github/workflows/platform-backend-ci.yml"
       - "autogpt_platform/backend/**"
-      - "autogpt_platform/autogpt_libs/**"
   merge_group:
 
 concurrency: