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/middleware.md

@@ -186,19 +186,19 @@ All middleware components follow a consistent naming pattern for maintainability
 ```
 ✅ Good:
 middleware/logging.py
-middleware/context.py
+middleware/store_context.py
 middleware/auth.py
 
 ❌ Avoid:
 middleware/logging_middleware.py
-middleware/context_middleware.py
+middleware/store_context_middleware.py
 middleware/auth_middleware.py
 ```
 
 **Rationale**:
 - Keeps names concise and consistent
 - Follows Django, Flask, and FastAPI conventions
-- Makes imports cleaner: `from middleware.logging import LoggingMiddleware`
+- Makes imports cleaner: `from middleware.store_context import StoreContextMiddleware`
 - Reduces redundancy (the `middleware/` directory already indicates the purpose)
 
 #### Test File Naming: Mirror the Source File
@@ -206,10 +206,9 @@ middleware/auth_middleware.py
 Test files directly mirror the middleware filename with a `test_` prefix:
 
 ```
-middleware/logging.py      → tests/unit/middleware/test_logging.py
-middleware/context.py      → tests/unit/middleware/test_context.py
-middleware/auth.py         → tests/unit/middleware/test_auth.py
+middleware/logging.py       → tests/unit/middleware/test_logging.py
 middleware/store_context.py → tests/unit/middleware/test_store_context.py
+middleware/auth.py          → tests/unit/middleware/test_auth.py
 ```
 
 #### One Component Per File
@@ -229,19 +228,15 @@ class FrontendTypeMiddleware:      # ASGI wrapper for frontend detection
 class AuthManager:                 # Authentication logic
 ```
 
-> **Note**: The old `middleware/context.py` with `ContextMiddleware` and `RequestContext` is deprecated.
-> Use `FrontendTypeMiddleware` and `FrontendType` enum instead.
-> See [Frontend Detection Architecture](frontend-detection.md) for migration guide.
-
 #### One Test File Per Component
 
 Follow the Single Responsibility Principle - each test file tests exactly one component:
 
 ```
 ✅ Good:
-tests/unit/middleware/test_logging.py      # Tests only LoggingMiddleware
-tests/unit/middleware/test_context.py      # Tests only ContextManager/Middleware
-tests/unit/middleware/test_decorators.py   # Tests only rate_limit decorator
+tests/unit/middleware/test_logging.py        # Tests only LoggingMiddleware
+tests/unit/middleware/test_store_context.py  # Tests only StoreContextManager/Middleware
+tests/unit/middleware/test_decorators.py     # Tests only rate_limit decorator
 
 ❌ Avoid:
 tests/unit/middleware/test_all_middleware.py  # Tests multiple components
@@ -261,12 +256,12 @@ When importing middleware components, use explicit imports:
 ```python
 # ✅ Preferred - Explicit and clear
 from middleware.logging import LoggingMiddleware
-from middleware.context import ContextManager, RequestContext
+from middleware.store_context import StoreContextManager
 from middleware.auth import AuthManager
 
 # ❌ Avoid - Less clear
 from middleware import logging_middleware
-from middleware import context_middleware
+from middleware import store_context_middleware
 ```
 
 **See**: [Complete Naming Conventions Guide](../development/naming-conventions.md) for project-wide standards.