phase 1: run-now backup — restic wrapper, job lifecycle, end-to-end

Lands the operator → server → agent → restic → server roundtrip for
on-demand backups. The flow:

  POST /api/hosts/{id}/jobs {kind:"backup",args:["/path"]}
    → server creates a queued Job row
    → server emits command.run over WS to the host's agent
    → agent dispatcher spawns runner.RunBackup in a goroutine
    → runner spawns `restic backup --json`, parses each line
    → forwards: job.started, log.stream (every line), job.progress
      (throttled to 1/sec), job.finished (with summary stats blob)
    → server WS handler persists those into jobs / job_logs

P1-16 internal/restic: thin Locate + Env wrapper that runs `restic
  backup --json`, scans stdout/stderr, parses BackupStatus +
  BackupSummary, calls back into a LineHandler so the agent can fan
  out to log.stream + job.progress. Treats exit code 3 as
  "succeeded with issues" (matches restic's contract).

P1-18 store: jobs accessors (CreateJob, MarkJobStarted,
  MarkJobFinished, AppendJobLog, GetJob).

P1-19 server: POST /api/hosts/{id}/jobs creates the Job row,
  validates kind, dispatches via Hub.Send, audit-logs the action.

P1-20 agent runner: wraps restic.RunBackup with throttled progress
  emission. Sender abstraction was added to wsclient.Handler so
  background goroutines can keep replying after dispatch returns.

P1-21 server WS: dispatchAgentMessage now persists job.started,
  job.finished, log.stream into the database. Browser fan-out for
  live tailing lands with the UI work.

Agent gets repo_url + repo_password from agent.yaml in plaintext
for now (mode 0600, owned by service user); spec.md §7.3's keyring
storage moves there in P2. config.update over WS overrides the
in-memory copy (does not persist).

Build clean; all tests pass. End-to-end with a real restic still
needs a host that has restic installed — wire shape verified by
the existing hello/heartbeat round-trip test.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/agent/wsclient/client.go

@@ -19,6 +19,7 @@ import (
 	stdhttp "net/http"
 	"net/url"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/coder/websocket"
@@ -36,10 +37,19 @@ type Config struct {
 	HelloPayload    api.HelloPayload
 }
 
-// Handler is invoked for every server-sent message. The agent's main
-// program supplies one that knows how to dispatch command.run etc.
-// to the runner package.
-type Handler func(ctx context.Context, env api.Envelope) error
+// Sender is what handlers use to push agent → server messages
+// (job.progress, job.finished, log.stream, command.result, …).
+// Returned by the WS client to the dispatch handler. Write operations
+// serialise behind a single mutex on the conn; concurrent calls are
+// safe.
+type Sender interface {
+	Send(env api.Envelope) error
+}
+
+// Handler is invoked for every server-sent message. tx lets the
+// handler push replies back; it is valid only for the lifetime of
+// the connection (calls fail if the agent has reconnected since).
+type Handler func(ctx context.Context, env api.Envelope, tx Sender) error
 
 // Run keeps the agent connected indefinitely. Returns when ctx is
 // cancelled. Errors during a single connection attempt are logged and
@@ -107,6 +117,8 @@ func connectOnce(ctx context.Context, cfg Config, handle Handler) error {
 	}
 	slog.Info("ws agent connected", "server", wsURL)
 
+	tx := &connSender{conn: conn, ctx: ctx}
+
 	// Heartbeat goroutine.
 	heartbeatCtx, cancelHeartbeat := context.WithCancel(ctx)
 	defer cancelHeartbeat()
@@ -138,13 +150,34 @@ func connectOnce(ctx context.Context, cfg Config, handle Handler) error {
 			continue
 		}
 		if handle != nil {
-			if err := handle(ctx, env); err != nil {
+			if err := handle(ctx, env, tx); err != nil {
 				slog.Warn("ws agent: handler returned error", "type", env.Type, "err", err)
 			}
 		}
 	}
 }
 
+// connSender is the per-connection Sender. Goroutines beyond the
+// read loop (e.g. a backup running in its own goroutine) keep a
+// reference to one of these for the duration of their work.
+type connSender struct {
+	conn *websocket.Conn
+	ctx  context.Context
+	mu   sync.Mutex
+}
+
+func (s *connSender) Send(env api.Envelope) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	raw, err := json.Marshal(env)
+	if err != nil {
+		return err
+	}
+	writeCtx, cancel := context.WithTimeout(s.ctx, 30*time.Second)
+	defer cancel()
+	return s.conn.Write(writeCtx, websocket.MessageText, raw)
+}
+
 func heartbeatLoop(ctx context.Context, conn *websocket.Conn, period time.Duration) {
 	t := time.NewTicker(period)
 	defer t.Stop()