refactor: tech debt reduction - extract hooks, deduplicate helpers, update deps, add CI

- Extract useCustomStepFlow hook from TreeNavigationPage (1040 → 759 lines)
- Create core/filters.py with shared tree/step visibility filters
- Create services/export_service.py from session export logic
- Add GitHub Actions CI/CD pipeline (pytest + lint + build)
- Add GIN index migration for full-text search on trees
- Update FastAPI 0.128.5, Pydantic 2.12.5, SQLAlchemy 2.0.46, +5 more
- Fix regex → pattern deprecation in Query() params

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/requirements.txt

@@ -1,22 +1,22 @@
 # FastAPI and server
-fastapi==0.109.2
-uvicorn[standard]==0.27.1
+fastapi==0.128.5
+uvicorn[standard]==0.40.0
 
 # Database
-sqlalchemy==2.0.25
-asyncpg==0.29.0
+sqlalchemy==2.0.46
+asyncpg==0.31.0
 psycopg2-binary==2.9.9
-alembic==1.13.1
+alembic==1.18.3
 
 # Authentication
 python-jose[cryptography]==3.3.0
 passlib[bcrypt]==1.7.4
 bcrypt==4.1.2
-python-multipart==0.0.9
+python-multipart==0.0.22
 
 # Validation and settings
-pydantic==2.6.1
-pydantic-settings==2.1.0
+pydantic==2.12.5
+pydantic-settings==2.12.0
 email-validator==2.1.0
 
 # Rate Limiting