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>

app/backend/Dockerfile

@@ -14,8 +14,8 @@ RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo \
     -ldflags "-X containr/internal/api.Version=${VERSION}" -o main cmd/server/main.go
 
 # Node-agent binaries served by the API for SSH/bootstrap installs.
-RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -o /agents/containr-agent-linux-amd64 ./cmd/agent && \
-    CGO_ENABLED=0 GOOS=linux GOARCH=arm64 go build -o /agents/containr-agent-linux-arm64 ./cmd/agent
+RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -ldflags "-X main.version=${VERSION}" -o /agents/containr-agent-linux-amd64 ./cmd/agent && \
+    CGO_ENABLED=0 GOOS=linux GOARCH=arm64 go build -ldflags "-X main.version=${VERSION}" -o /agents/containr-agent-linux-arm64 ./cmd/agent
 
 # Build embedded Better Auth runtime dependencies
 FROM node:22-alpine AS auth-builder