feat: dashboard screenshot upload & retention (last 20 per client)

- Listener: subscribe to dashboard topic, forward screenshots to API
- API: store latest + last 20 timestamped screenshots per client, auto-delete older files
- Docs: updated README, TECH-CHANGELOG, and copilot-instructions for screenshot upload and retention policy

SCREENSHOT_IMPLEMENTATION.md

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+# Screenshot Transmission Implementation
+
+## Overview
+Clients send screenshots via MQTT during heartbeat intervals. The listener service receives these screenshots and forwards them to the server API for storage.
+
+## Architecture
+
+### MQTT Topic
+- **Topic**: `infoscreen/{uuid}/screenshot`
+- **Payload Format**: 
+  - Raw binary image data (JPEG/PNG), OR
+  - JSON with base64-encoded image: `{"image": "<base64-string>"}`
+
+### Components
+
+#### 1. Listener Service (`listener/listener.py`)
+- **Subscribes to**: `infoscreen/+/screenshot`
+- **Function**: `handle_screenshot(uuid, payload)`
+  - Detects payload format (binary or JSON)
+  - Converts binary to base64 if needed
+  - Forwards to API via HTTP POST
+
+#### 2. Server API (`server/routes/clients.py`)
+- **Endpoint**: `POST /api/clients/<uuid>/screenshot`
+- **Authentication**: No authentication required (internal service call)
+- **Accepts**: 
+  - JSON: `{"image": "<base64-encoded-image>"}`
+  - Binary: raw image data
+- **Storage**: 
+  - Saves to `server/screenshots/{uuid}_{timestamp}.jpg` (with timestamp)
+  - Saves to `server/screenshots/{uuid}.jpg` (latest, for quick retrieval)
+
+#### 3. Retrieval (`server/wsgi.py`)
+- **Endpoint**: `GET /screenshots/<uuid>`
+- **Returns**: Latest screenshot for the given client UUID
+- **Nginx**: Exposes `/screenshots/{uuid}.jpg` in production
+
+## Unified Identification Method
+
+Screenshots are identified by **client UUID**:
+- Each client has a unique UUID stored in the `clients` table
+- Screenshots are stored as `{uuid}.jpg` (latest) and `{uuid}_{timestamp}.jpg` (historical)
+- The API endpoint requires UUID validation against the database
+- Retrieval is done via `GET /screenshots/<uuid>` which returns the latest screenshot
+
+## Data Flow
+
+```
+Client → MQTT (infoscreen/{uuid}/screenshot)
+    ↓
+Listener Service
+    ↓ (validates client exists)
+    ↓ (converts binary → base64 if needed)
+    ↓
+API POST /api/clients/{uuid}/screenshot
+    ↓ (validates client UUID)
+    ↓ (decodes base64 → binary)
+    ↓
+Filesystem: server/screenshots/{uuid}.jpg
+    ↓
+Dashboard/Nginx: GET /screenshots/{uuid}
+```
+
+## Configuration
+
+### Environment Variables
+- **Listener**: `API_BASE_URL` (default: `http://server:8000`)
+- **Server**: Screenshots stored in `server/screenshots/` directory
+
+### Dependencies
+- Listener: Added `requests>=2.31.0` to `listener/requirements.txt`
+- Server: Uses built-in Flask and base64 libraries
+
+## Error Handling
+
+- **Client Not Found**: Returns 404 if UUID doesn't exist in database
+- **Invalid Payload**: Returns 400 if image data is missing or invalid
+- **API Timeout**: Listener logs error and continues (timeout: 10s)
+- **Network Errors**: Listener logs and continues operation
+
+## Security Considerations
+
+- Screenshot endpoint does not require authentication (internal service-to-service)
+- Client UUID must exist in database before screenshot is accepted
+- Base64 encoding prevents binary data issues in JSON transport
+- File size is tracked and logged for monitoring
+
+## Future Enhancements
+
+- Add screenshot retention policy (auto-delete old timestamped files)
+- Add compression before transmission
+- Add screenshot quality settings
+- Add authentication between listener and API
+- Add screenshot history API endpoint