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_platform_context.py

@@ -22,6 +22,7 @@ import pytest
 from fastapi import Request
 from sqlalchemy.orm import Session
 
+from app.core.frontend_detector import FrontendDetector
 from middleware.platform_context import (
     DEFAULT_PLATFORM_CODE,
     PlatformContextManager,
@@ -210,7 +211,7 @@ class TestPlatformContextManager:
         request.headers = {"host": "admin.localhost"}
         request.url = Mock(path="/dashboard")
 
-        assert PlatformContextManager.is_admin_request(request) is True
+        assert FrontendDetector.is_admin("admin.localhost", "/dashboard") is True
 
         context = PlatformContextManager.detect_platform_context(request)
         assert context is None
@@ -221,26 +222,18 @@ class TestPlatformContextManager:
         request.headers = {"host": "localhost"}
         request.url = Mock(path="/admin/stores")
 
-        assert PlatformContextManager.is_admin_request(request) is True
+        assert FrontendDetector.is_admin("localhost", "/admin/stores") is True
 
         context = PlatformContextManager.detect_platform_context(request)
         assert context is None
 
     def test_skip_admin_path_with_port(self):
         """Test admin detection with port in host."""
-        request = Mock(spec=Request)
-        request.headers = {"host": "admin.localhost:9999"}
-        request.url = Mock(path="/dashboard")
-
-        assert PlatformContextManager.is_admin_request(request) is True
+        assert FrontendDetector.is_admin("admin.localhost:9999", "/dashboard") is True
 
     def test_not_admin_regular_path(self):
         """Test non-admin path is not detected as admin."""
-        request = Mock(spec=Request)
-        request.headers = {"host": "localhost"}
-        request.url = Mock(path="/shop/products")
-
-        assert PlatformContextManager.is_admin_request(request) is False
+        assert FrontendDetector.is_admin("localhost", "/shop/products") is False
 
     # ========================================================================
     # Static File Detection Tests
@@ -925,11 +918,7 @@ class TestEdgeCases:
 
     def test_admin_subdomain_with_production_domain(self):
         """Test admin subdomain detection for production domains."""
-        request = Mock(spec=Request)
-        request.headers = {"host": "admin.oms.lu"}
-        request.url = Mock(path="/dashboard")
-
-        assert PlatformContextManager.is_admin_request(request) is True
+        assert FrontendDetector.is_admin("admin.oms.lu", "/dashboard") is True
 
     def test_static_file_case_insensitive(self):
         """Test static file detection is case-insensitive."""