Fix timer action 500s after FastAPI upgrade

FastAPI 0.141 raises when response serialization hits a lazy-load
(current_block.subject.options) in async context, where 0.115 silently
skipped it. Eager-load Subject.options in session get/timer and the TV
dashboard queries. Pin pydantic 2.13.5.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

backend/requirements.txt

@@ -9,6 +9,7 @@ PyJWT==2.15.0
 cryptography==50.0.1
 passlib[bcrypt]==1.7.4
 bcrypt==3.2.2
+pydantic==2.13.5
 pydantic-settings==2.5.2
 alembic==1.20.0
 Mako==1.4.3