fix(deps): relax pydantic pin to >=2.12.5 to satisfy litellm 1.83.10

litellm 1.83.10 requires pydantic==2.12.5, but requirements.txt had
pydantic==2.11.3 hard-pinned, causing pip ResolutionImpossible on
fresh CI installs. Local venv had pydantic from earlier session, so
the conflict was masked.

fastapi 0.115.12 (>=1.7.4, <3.0.0) and pydantic-settings 2.9.1
(>=2.7.0) both accept 2.12.5, so range >=2.12.5,<3 satisfies all.

Local 154 tests still pass with bumped pydantic.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: CreatmanCEO

backend/requirements.txt

@@ -1,6 +1,6 @@
 fastapi==0.115.12
 uvicorn[standard]==0.34.2
-pydantic==2.11.3
+pydantic>=2.12.5,<3
 pydantic-settings==2.9.1
 litellm==1.83.10
 instructor[google-genai]==1.15.1