refactor(status): introduce OrderStatus enum, replace string literals

Centralized order status values in api/app/constants.py via a
str-valued Enum so comparisons keep working. Replaced literals in:
- services: sync_service, sqlite_service, retry_service
- routers: sync, dashboard
- templates: dashboard.html, logs.html
- static JS: shared (ORDER_STATUS mirror), dashboard, logs
- tests: requirements, order_items_overwrite, business_rules

MALFORMED intentionally NOT added — introduced in follow-up PR2
(per-order failure isolation).

Full test suite: 231 unit + 33 e2e pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/app/constants.py

@@ -0,0 +1,17 @@
+"""Application-wide constants shared across services, routers, and tests."""
+from enum import Enum
+
+
+class OrderStatus(str, Enum):
+    """Order status values stored in SQLite `orders.status` column.
+
+    Inherits from `str` so existing string comparisons (==, in, dict.get)
+    keep working. Always use `.value` when passing to SQL queries or JSON
+    payloads to avoid Python-version-specific str(enum) surprises.
+    """
+    IMPORTED = "IMPORTED"
+    ALREADY_IMPORTED = "ALREADY_IMPORTED"
+    SKIPPED = "SKIPPED"
+    ERROR = "ERROR"
+    CANCELLED = "CANCELLED"
+    DELETED_IN_ROA = "DELETED_IN_ROA"