Persist operation state in server and reuse returned snapshots

docs/product/application-state.md

@@ -0,0 +1,63 @@
+# Harbor application state v1
+
+`GET /api/state` is the canonical Harbor domain snapshot. Successful POST and DELETE endpoints return the same snapshot as `state` while retaining their v0 response fields for compatibility.
+
+```json
+{
+  "apiVersion": 1,
+  "revision": 42,
+  "generatedAt": "2026-07-11T15:00:00.000Z",
+  "mode": "client",
+  "subscription": {
+    "status": "ready",
+    "host": "provider.example/…",
+    "fetchedAt": "2026-07-11T14:58:00.000Z",
+    "userInfo": {}
+  },
+  "selection": {
+    "desiredServerId": "Amsterdam",
+    "appliedServerId": "Amsterdam"
+  },
+  "connection": {
+    "desired": "running",
+    "process": "running",
+    "startedAt": "2026-07-11T14:59:10.000Z",
+    "lastError": null
+  },
+  "route": {
+    "mode": "local-vpn",
+    "gatewayAddress": null,
+    "lastVerifiedAt": null,
+    "reason": "auto"
+  },
+  "operation": {
+    "kind": null,
+    "status": "idle",
+    "startedAt": null,
+    "error": null
+  },
+  "servers": []
+}
+```
+
+The backend owns subscription metadata, servers, desired/applied selection, desired/process connection state, route and current operation. React may keep only unsaved form values, pending selection and visual state. Browser transport freshness is not part of this contract.
+
+## Revision rules
+
+`revision` is persisted in the existing `state.json` and increases on externally visible transitions, including operation start/completion/failure, import, refresh, forget, apply, start, stop and Gateway Auto changes. `generatedAt` is response metadata and does not change revision by itself.
+
+A consumer must eventually apply only snapshots whose revision is at least its current revision. The frontend comparison and stale/offline transport envelope are intentionally handled by TASK-002 and TASK-003.
+
+## Desired and applied state
+
+`selection.desiredServerId` records the user's requested server. `selection.appliedServerId` changes only after its sing-box configuration has been applied. Likewise, `connection.desired` records intent while `connection.process` reports the observed runtime. A failed operation can therefore leave desired and applied values different without pretending that the request succeeded.
+
+Server IDs are currently derived from the existing trimmed subscription tag. Stable IDs across cosmetic renames are deferred to TASK-008.
+
+## Compatibility and migration
+
+No path, volume or file is renamed. A legacy `state.json` without `revision`, `appliedTag` or `connectionDesired` is normalized on read: revision starts at `0`, the legacy `selectedTag` is treated as both desired and applied, and connection intent is inferred from the existing config. The next domain write stores the added fields. Existing unknown fields remain untouched.
+
+During the v0 compatibility window, the snapshot also exposes `selectedTag`, `singboxRunning`, `servers[].tag`, `gatewayAuto` and the other previous GET fields. Mutation responses retain their previous result fields and add `state`. The canonical `subscription` object never contains the full subscription URL.
+
+Rollback is code-only: deploy the previous build. The added persisted fields are ignored by the previous implementation, so no data rewrite is needed. Revisions created by the newer build remain harmless integers in `state.json`.