refactor: remove all backward compatibility code across 70 files

Clean up 28 backward compatibility instances identified in the codebase.
The app is not live, so all shims are replaced with the target architecture:

- Remove legacy Inventory.location column (use bin_location exclusively)
- Remove dashboard _extract_metric_value helper (use flat metrics dict)
- Remove legacy stat field duplicates (total_stores, total_imports, etc.)
- Remove 13 re-export shims and class aliases across modules
- Remove module-enabling JSON fallback (use PlatformModule junction table)
- Remove menu_to_legacy_format() conversion (return dataclasses directly)
- Remove title/description from MarketplaceProductBase schema
- Clean billing convenience method docstrings
- Clean test fixtures and backward-compat comments
- Add PlatformModule seeding to init_production.py

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

docs/architecture/multi-platform-cms.md

@@ -123,7 +123,7 @@ The system uses different URL patterns for development vs production:
 
 **Production (custom domains):**
 - Main marketing site: `orion.lu/` → `main` platform
-- Platform sites: `oms.lu/`, `loyalty.lu/` → specific platform
+- Platform sites: `omsflow.lu/`, `rewardflow.lu/` → specific platform
 
 ### Request Processing
 
@@ -259,7 +259,7 @@ Request: GET /about
 1. Insert platform record:
    ```sql
    INSERT INTO platforms (code, name, description, domain, path_prefix)
-   VALUES ('loyalty', 'Loyalty Program', 'Customer loyalty and rewards', 'loyalty.lu', 'loyalty');
+   VALUES ('loyalty', 'Loyalty Program', 'Customer loyalty and rewards', 'rewardflow.lu', 'loyalty');
    ```
 
 2. Create platform-specific content pages:
@@ -270,7 +270,7 @@ Request: GET /about
 
 3. Configure routing:
    - **Development:** Access at `localhost:9999/platforms/loyalty/`
-   - **Production:** Access at `loyalty.lu/`
+   - **Production:** Access at `rewardflow.lu/`
    - Platform detected automatically by `PlatformContextMiddleware`
    - No additional route configuration needed
 
@@ -285,5 +285,5 @@ Request: GET /about
 | Platform | Code | Dev URL | Prod URL |
 |----------|------|---------|----------|
 | Main Marketing | `main` | `localhost:9999/` | `orion.lu/` |
-| OMS | `oms` | `localhost:9999/platforms/oms/` | `oms.lu/` |
-| Loyalty | `loyalty` | `localhost:9999/platforms/loyalty/` | `loyalty.lu/` |
+| OMS | `oms` | `localhost:9999/platforms/oms/` | `omsflow.lu/` |
+| Loyalty | `loyalty` | `localhost:9999/platforms/loyalty/` | `rewardflow.lu/` |