feat: add connections dashboard with background health checks

- Add /connections page showing all integrations with online/offline status
- Add background health checker that polls Evotor API every 10 minutes
- Add is_online and last_checked_at fields to evotor_connections table
- Replace Evotor navbar link with unified Connections link
- Redirect connect/disconnect flows to /connections
- Add Alembic migration for new columns

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

docs/plans/connections-dashboard.md

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+# Connections Dashboard with Background Health Checks
+
+## Context
+
+Users currently access Evotor connection via a dedicated `/evotor` page linked from the navbar. As more integrations are planned, we need a unified **Connections** page where users can see all their connections at a glance, with real-time status indicators powered by background health checks.
+
+## Plan
+
+### 1. Model Changes — `web/models.py`
+
+Add to `EvotorConnection`:
+- `is_online` (Boolean, default=False, server_default="0")
+- `last_checked_at` (DateTime, nullable)
+
+### 2. Alembic Migration
+
+Generate migration for the two new columns on `evotor_connections` table.
+
+### 3. Config Addition — `web/config.py`
+
+Add `HEALTH_CHECK_INTERVAL_SECONDS: int = 600` (10 minutes default).
+
+### 4. Background Health Checker — `web/health_checker.py` (new)
+
+- `check_evotor_connection(access_token) -> bool` — async, calls `GET https://api.evotor.ru/stores` with Bearer token, returns True if 200
+- `run_health_checks()` — queries all `EvotorConnection` rows using its own `SessionLocal()`, checks each, updates `is_online` and `last_checked_at`
+- `health_check_loop(interval)` — infinite loop with `asyncio.sleep`, calls `run_health_checks()`
+
+### 5. Wire Background Task — `web/main.py`
+
+Add FastAPI lifespan context manager:
+- On startup: `asyncio.create_task(health_check_loop(...))`
+- On shutdown: cancel the task
+- Register new connections router
+
+### 6. Connections Route — `web/routes/connections.py` (new)
+
+`GET /connections` — requires auth, builds a list of connection descriptors:
+```python
+connections = [{
+    "name": "Эвотор",
+    "icon": "bi-shop",
+    "connected": bool,
+    "is_online": bool,
+    "last_checked_at": datetime | None,
+    "details": store_name,
+    "connect_url": "/evotor",
+    "disconnect_url": "/evotor/disconnect",
+}]
+```
+Future connections just append another dict — template stays generic.
+
+### 7. Connections Template — `web/templates/connections.html` (new)
+
+Card per connection showing:
+- Icon + name + optional details (store name)
+- Status: green `bi-circle-fill` (online), red `bi-circle-fill` (offline), grey `bi-circle` (not connected)
+- Action button: "Подключить" (link to connect_url) or "Отключить" (POST form to disconnect_url)
+- Card footer with last check timestamp
+
+### 8. Navbar Update — `web/templates/base.html`
+
+Replace "Эвотор" link with "Подключения" → `/connections`.
+
+### 9. Evotor Callback Update — `web/routes/evotor.py`
+
+On successful OAuth callback, set `is_online=True` and `last_checked_at=func.now()`.
+
+### 10. Evotor Template Back Link — `web/templates/evotor.html`
+
+Change "Вернуться в личный кабинет" → "Вернуться к подключениям" linking to `/connections`.
+
+## Files Summary
+
+| File | Action |
+|------|--------|
+| `web/models.py` | Modify — add 2 fields |
+| `web/config.py` | Modify — add interval setting |
+| `web/main.py` | Modify — lifespan + router |
+| `web/routes/evotor.py` | Modify — set online on callback |
+| `web/routes/connections.py` | Create |
+| `web/health_checker.py` | Create |
+| `web/templates/connections.html` | Create |
+| `web/templates/base.html` | Modify — navbar |
+| `web/templates/evotor.html` | Modify — back link |
+| Alembic migration | Create |
+
+## Verification
+
+1. Run `alembic upgrade head` to apply migration
+2. Start the app, verify background task logs appear
+3. Visit `/connections` — should show Evotor as disconnected (grey)
+4. Connect Evotor via `/evotor` — should redirect back, connections page shows green status
+5. Disconnect — status returns to grey
+6. Wait for health check interval or trigger manually — verify `is_online` and `last_checked_at` update