Add REST API endpoints for games, runs, and encounters

Implement 13 endpoints: read-only reference data (games, routes, pokemon),
run CRUD with cascading deletes, and encounter management. Uses Pydantic v2
with camelCase alias generation to match frontend types, and nested response
schemas for detail views.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/app/schemas/base.py

@@ -0,0 +1,10 @@
+from pydantic import BaseModel, ConfigDict
+from pydantic.alias_generators import to_camel
+
+
+class CamelModel(BaseModel):
+    model_config = ConfigDict(
+        from_attributes=True,
+        alias_generator=to_camel,
+        populate_by_name=True,
+    )