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>

docs/architecture/frontend-detection.md

@@ -172,41 +172,6 @@ if FrontendDetector.is_storefront(host, path, has_store_context=True):
 | Custom domain | mybakery.lu | /products | STOREFRONT |
 | Platform root | oms.lu | /pricing | PLATFORM |
 
-## Migration from RequestContext
-
-The previous `RequestContext` enum is deprecated. Here's the mapping:
-
-| Old (RequestContext) | New (FrontendType) |
-|---------------------|-------------------|
-| `API` | Use `FrontendDetector.is_api_request()` + FrontendType |
-| `ADMIN` | `FrontendType.ADMIN` |
-| `STORE_DASHBOARD` | `FrontendType.STORE` |
-| `SHOP` | `FrontendType.STOREFRONT` |
-| `FALLBACK` | `FrontendType.PLATFORM` |
-
-### Code Migration
-
-**Before (deprecated):**
-```python
-from middleware.context import RequestContext, get_request_context
-
-context = get_request_context(request)
-if context == RequestContext.SHOP:
-    # Storefront logic
-    pass
-```
-
-**After:**
-```python
-from middleware.frontend_type import get_frontend_type
-from app.modules.enums import FrontendType
-
-frontend_type = get_frontend_type(request)
-if frontend_type == FrontendType.STOREFRONT:
-    # Storefront logic
-    pass
-```
-
 ## Request State
 
 After `FrontendTypeMiddleware` runs, the following is available:
@@ -255,10 +220,9 @@ pytest tests/unit/core/test_frontend_detector.py tests/unit/middleware/test_fron
 
 ### DON'T
 
-1. **Don't use `RequestContext`** - it's deprecated
-2. **Don't duplicate path detection logic** - use FrontendDetector
-3. **Don't hardcode path patterns** in middleware - they're centralized in FrontendDetector
-4. **Don't check `request.state.context_type`** - use `request.state.frontend_type`
+1. **Don't duplicate path detection logic** - use FrontendDetector
+2. **Don't hardcode path patterns** in middleware - they're centralized in FrontendDetector
+3. **Don't check `request.state.context_type`** - use `request.state.frontend_type`
 
 ## Architecture Rules