feat: implement API versioning /api/v1/

- All backend routers moved to /api/v1/ prefix
- Frontend BASE_URL updated to /api/v1
- Setup redirect middleware updated to redirect to /api/v1/setup
- Health router path fixed: prefix=/api/v1/health, @router.get('')
- conftest.py: set server_status=online for test fixture
- Created Docs/API_VERSIONING.md with deprecation policy
- Updated Docs/Backend-Development.md with versioning section
- Updated Instructions.md curl examples

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/routers/geo.py

@@ -21,7 +21,7 @@ from app.dependencies import (
 from app.models.geo import GeoCacheStatsResponse, GeoReResolveResponse, IpLookupResponse
 from app.services import geo_service, jail_service
 
-router: APIRouter = APIRouter(prefix="/api/geo", tags=["Geo"])
+router: APIRouter = APIRouter(prefix="/api/v1/geo", tags=["Geo"])
 
 _IpPath = Annotated[str, Path(description="IPv4 or IPv6 address to look up.")]