refactor: remove backward compatibility code for pre-launch baseline

Clean up accumulated backward-compat shims, deprecated wrappers, unused
aliases, and legacy code across the codebase. Since the platform is not
live yet, this establishes a clean baseline.

Changes:
- Delete deprecated middleware/context.py (RequestContext, get_request_context)
- Remove unused factory get_store_email_settings_service()
- Remove deprecated pagination_full macro, /admin/platform-homepage route
- Remove ConversationResponse, InvoiceSettings* unprefixed aliases
- Simplify celery_config.py (remove empty LEGACY_TASK_MODULES)
- Standardize billing exceptions: *Error aliases → *Exception names
- Consolidate duplicate TierNotFoundError/FeatureNotFoundError classes
- Remove deprecated is_admin_request() from Store/PlatformContextManager
- Remove is_platform_default field, MediaUploadResponse legacy flat fields
- Remove MediaItemResponse.url alias, update JS to use file_url
- Update all affected tests and documentation

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

tests/unit/middleware/test_store_context.py

@@ -17,6 +17,7 @@ import pytest
 from fastapi import Request
 from sqlalchemy.orm import Session
 
+from app.core.frontend_detector import FrontendDetector
 from app.modules.tenancy.exceptions import StoreNotFoundException
 from middleware.store_context import (
     StoreContextManager,
@@ -372,35 +373,19 @@ class TestStoreContextManager:
 
     def test_is_admin_request_admin_subdomain(self):
         """Test admin request detection from subdomain."""
-        request = Mock(spec=Request)
-        request.headers = {"host": "admin.platform.com"}
-        request.url = Mock(path="/dashboard")
-
-        assert StoreContextManager.is_admin_request(request) is True
+        assert FrontendDetector.is_admin("admin.platform.com", "/dashboard") is True
 
     def test_is_admin_request_admin_path(self):
         """Test admin request detection from path."""
-        request = Mock(spec=Request)
-        request.headers = {"host": "localhost"}
-        request.url = Mock(path="/admin/dashboard")
-
-        assert StoreContextManager.is_admin_request(request) is True
+        assert FrontendDetector.is_admin("localhost", "/admin/dashboard") is True
 
     def test_is_admin_request_with_port(self):
         """Test admin request detection with port number."""
-        request = Mock(spec=Request)
-        request.headers = {"host": "admin.localhost:8000"}
-        request.url = Mock(path="/dashboard")
-
-        assert StoreContextManager.is_admin_request(request) is True
+        assert FrontendDetector.is_admin("admin.localhost:8000", "/dashboard") is True
 
     def test_is_not_admin_request(self):
         """Test non-admin request."""
-        request = Mock(spec=Request)
-        request.headers = {"host": "store1.platform.com"}
-        request.url = Mock(path="/storefront")
-
-        assert StoreContextManager.is_admin_request(request) is False
+        assert FrontendDetector.is_admin("store1.platform.com", "/storefront") is False
 
     def test_is_api_request(self):
         """Test API request detection."""
@@ -599,7 +584,7 @@ class TestStoreContextMiddleware:
 
         call_next = AsyncMock(return_value=Mock())
 
-        with patch.object(StoreContextManager, "is_admin_request", return_value=True):
+        with patch.object(FrontendDetector, "is_admin", return_value=True):
             await middleware.dispatch(request, call_next)
 
         assert request.state.store is None
@@ -775,7 +760,7 @@ class TestStoreContextMiddleware:
         call_next = AsyncMock(return_value=Mock())
 
         with (
-            patch.object(StoreContextManager, "is_admin_request", return_value=False),
+            patch.object(FrontendDetector, "is_admin", return_value=False),
             patch.object(
                 StoreContextManager, "is_static_file_request", return_value=False
             ),