docs: ADR for publishing a RabbitMQ settings update message

Author: shepilov

docs/adr/003-rabbitmq-settings-publishing.md

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+# ADR: Replace Common Settings HTTP API with RabbitMQ Publishing
+
+## Status
+
+**Proposed**
+
+## Context
+
+The cozy-stack currently synchronizes user settings with an external "common settings" application via HTTP API calls. 
+This integration is implemented in `model/settings/common/common.go` and performs:
+
+1. **HTTP POST** to `/api/admin/user/settings` when creating settings
+2. **HTTP GET** to `/api/admin/user/settings/{nickname}` to check remote version
+3. **HTTP PUT** to `/api/admin/user/settings/{nickname}` to update settings
+
+This approach has several issues:
+
+- **Synchronous blocking**: HTTP calls block the settings update operation
+- **Version conflict complexity**: The code checks remote versions before updates, creating complex conflict resolution logic
+- **Tight coupling**: Direct HTTP dependency on the common settings app
+- **Failure propagation**: HTTP failures can affect the main settings update flow
+- **No longer needed**: The common settings app is no longer actively updating settings, making version conflict checking obsolete
+
+The codebase already has a mature RabbitMQ infrastructure for message consumption (password updates, user creation, phone updates, subscription changes, app lifecycle). 
+This infrastructure can be extended for publishing.
+
+## Proposal
+
+Replace the HTTP API calls with RabbitMQ message publishing using a fire-and-forget pattern:
+
+### Architecture
+
+```mermaid
+flowchart LR
+    subgraph CozyStack["cozy-stack"]
+        Publisher["Publisher"]
+    end
+
+    subgraph Storage["Storage"]
+        CouchDB["CouchDB<br/>(settings)"]
+    end
+
+    subgraph MessageBroker["Message Broker"]
+        RabbitMQ["RabbitMQ<br/>(exchange: settings)"]
+    end
+
+    subgraph Consumer["TWP app"]
+        CommonSettings["Common Settings<br/>(consume)"]
+    end
+
+    Publisher -->|"1. Update settings"| CouchDB
+    Publisher -->|"2. Publish message"| RabbitMQ
+    RabbitMQ -->|"user.settings.updated"| CommonSettings
+```
+
+### Key Changes
+
+1. **New Publisher Infrastructure** (`pkg/rabbitmq/publisher.go`)
+   - Reuses existing `RabbitMQConnection` for connection management
+   - Provides thread-safe publishing with automatic channel recovery
+   - JSON serialization with persistent delivery mode
+
+2. **Service Layer Extension** (`pkg/rabbitmq/service.go`)
+   - Add publishing capabilities alongside existing consumers
+   - Context-aware publisher selection (same as consumers)
+
+3. **Simplified Common Settings Module** (`model/settings/common/common.go`)
+   - Replace HTTP calls with RabbitMQ publishing
+   - Remove version conflict checking (local DB is source of truth)
+   - Fire-and-forget: log errors but don't fail the operation
+
+### Configuration
+
+```yaml
+rabbitmq:
+  enabled: true
+  nodes:
+    default:
+      url: amqp://guest:guest@localhost:5672/
+  publishing:
+    common_settings:
+      exchange: "settings"
+      routing_key: "user.settings.updated"
+```
+
+### Message Format
+
+Preserve existing message structure for backward compatibility:
+
+```json
+{
+  "source": "cozy-stack",
+  "nickname": "user-slug",
+  "request_id": "domain_1234567890",
+  "timestamp": 1234567890,
+  "version": 2,
+  "payload": {
+    "language": "en",
+    "timezone": "Europe/Paris",
+    "first_name": "John",
+    "last_name": "Doe",
+    "display_name": "John Doe",
+    "email": "john@example.com",
+    "phone": "+33612345678",
+    "matrix_id": "@john:example.com",
+    "avatar": "https://cozy.example.com/public/avatar?v=2"
+  }
+}
+```
+
+## Alternatives
+
+## Decision
+
+## Consequences
+