feat(nodes): agent self-upgrade (#57)

POST /agents/:id/upgrade enqueues a self_upgrade command carrying the
target platform, the sha256 the server computed over its bundled binary,
and the server version. The agent downloads the replacement from the
existing /api/agents/download/:platform endpoint, verifies the checksum,
sanity-checks the new binary (--version), atomically swaps its own
executable, reports the command result, then re-execs via syscall.Exec —
no supervisor or restart plumbing required.

Agents stamp main.version via ldflags at image build, expose --version,
and now send version at register so node_agents.version refreshes on
re-register (older agents without the field keep their stored value).
The server rejects upgrades for offline nodes and for platforms with no
bundled binary (linux-amd64/linux-arm64 ship in the image).

Surfaces: nodes upgrade CLI, containr_nodes_upgrade MCP (confirm-gated),
node-detail page button, OpenAPI. Verified live: v0.0.3 agent upgraded
to v0.0.4 in place, re-registered with the new version; offline node
returns 409 NODE_OFFLINE.

Generated with [Devin](https://devin.ai)

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

docs/api/openapi.yaml

@@ -5503,6 +5503,46 @@ paths:
               schema:
                 $ref: '#/components/schemas/Error'
 
+  /agents/{id}/upgrade:
+    post:
+      tags:
+        - Agents
+      summary: Upgrade the node agent
+      description: >-
+        Enqueues a `self_upgrade` command. The agent downloads its
+        replacement binary from this server's bundled `/agents/download`
+        endpoint, verifies the sha256, atomically swaps the executable, and
+        re-execs itself — the command result reports before the process
+        swaps. The node must be online, and the server must bundle a binary
+        for the node's architecture (`linux-amd64`/`linux-arm64`).
+      security:
+        - bearerAuth: []
+      parameters:
+        - $ref: '#/components/parameters/AgentIdParam'
+      responses:
+        '202':
+          description: Upgrade command enqueued
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  command: { type: object }
+                  platform: { type: string }
+                  version: { type: string }
+        '404':
+          description: Agent not found
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Error'
+        '409':
+          description: Node offline or no bundled binary for its architecture
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Error'
+
   /agents/{id}/metrics:
     get:
       tags: