Import documentation: docs/frontend-api-contracts.md

Import the existing local project snapshot. This commit organizes existing files; it does not represent newly developed functionality.

docs/frontend-api-contracts.md

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+# Frontend API Contracts
+
+Contracts are defined in `src/api/contracts`.
+
+Every service returns:
+
+```json
+{
+  "data": {},
+  "meta": {
+    "requestId": "req_..."
+  },
+  "error": null
+}
+```
+
+Errors return:
+
+```json
+{
+  "data": null,
+  "meta": {
+    "requestId": "req_..."
+  },
+  "error": {
+    "code": "VALIDATION_ERROR",
+    "message": "Human-readable message",
+    "fields": {}
+  }
+}
+```
+
+Shared DTOs cover pagination, filtering, sorting, bulk operations, jobs, job progress, audit metadata, permission checks, and WebSocket events.