P1 polish: agent-as-root, init-repo flow, rest creds passthrough, UX fixes

Cohesive batch from a smoke-test session against a real rest-server.
Themed bullets:

* Agent runs as root, sandboxed via systemd. CapabilityBoundingSet
  drops to CAP_DAC_READ_SEARCH + restore caps; ProtectSystem=strict
  with ReadWritePaths confined to /etc + /var/lib/restic-manager;
  NoNewPrivileges blocks escalation. Install script no longer
  creates a service user. spec.md §4.2 / §14.1 / §14.3 explain the
  rationale (matches UrBackup / Veeam / Bareos defaults; trying to
  back up "everything" as an unprivileged user creates silent skips
  on /home, /root, /var/lib/* with no upside vs the threat model
  the agent already implies).

* Init-repo end-to-end. New JobKind="init" wired through agent
  runner, restic.Env.RunInit, server dispatcher, and a UI button
  (red "Initialise repo" in the run-now panel). hosts.repo_initialised_at
  flips on init success, on backup success, or on a non-empty
  snapshots.report. The "Run now" / "Init" / "Retry" branching now
  drives both the dashboard host row and the host-detail panel.
  Migrations 0004 (column), 0005 (jobs.kind CHECK widened — using
  the safe create-new-then-rename pattern; first version corrupted
  job_logs.job_id FK), 0006 (cleans up job_logs FK on already-
  affected DBs).

* rest-server creds embedded at exec time only. restic.Env gains
  RepoUsername; mergeRestCreds() builds the user:pass@-prefixed URL
  inside envSlice() and never assigns it back to the struct, so
  nothing slog-able ever sees the cleartext form. RedactURL helper
  for any future surface that needs to log a URL safely. Both
  helpers tested.

* Add-host UX. Repo password is now optional — server mints a
  24-byte URL-safe random one and surfaces it once, alongside an
  htpasswd snippet ("echo PASS | htpasswd -B -i ... USERNAME") so
  the operator pastes one command on the rest-server host and one
  on the endpoint. Result page also links the install snippet at
  /install/install.sh (was /install.sh — 404'd before) and pipes
  to bash (not sh — script uses set -o pipefail and other
  bashisms; on Debian/Ubuntu sh is dash).

* Late-subscriber race in JobHub. A fast-failing job could finish
  (DB write + Broadcast) before the browser's HX-Redirect → page
  load → WS-connect path completed, so the JS sat forever waiting
  on a job.finished that already passed. JobHub split into
  Register + Send + Run; handleJobStream now subscribes first,
  re-fetches the job, and sends a synthetic job.finished if the
  state is already terminal.

* HTMX error visibility. New toast partial listens to
  htmx:responseError and surfaces the response body as a
  bottom-right toast — every server-side validation error now
  becomes visible without per-handler JS wiring. Also handles
  custom rm:toast events for future server-pushed notifications
  via the HX-Trigger header. Themed via existing CSS vars.

* Dashboard rows are now whole-row clickable to host detail
  (CSS card-link pattern: absolute-positioned anchor + .row-action
  z-index restoration so the action button stays clickable).
  "View →" on a running job links to /jobs/<id> rather than
  /hosts/<id> since the row click already covers the host page.

* "Run first" / "Run first backup" → "Run now" everywhere for
  consistency.

* runbook (docs/e2e-smoke.md) updated — live-log streaming step
  now reflects P1-26; mentions the browser-driven Run-now flow.

* _diag/dump-creds — moved out of cmd/ so go build doesn't pick
  it up; .gitignore now excludes /_diag/ entirely.

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

cmd/agent/main.go

@@ -243,6 +243,7 @@ func (d *dispatcher) runJob(ctx context.Context, p api.CommandRunPayload, tx wsc
 	r := runner.New(runner.Config{
 		ResticBin:    d.resticBin,
 		RepoURL:      creds.URL,
+		RepoUsername: creds.Username,
 		RepoPassword: creds.Password,
 	}, tx, time.Second)
 
@@ -259,6 +260,15 @@ func (d *dispatcher) runJob(ctx context.Context, p api.CommandRunPayload, tx wsc
 			}
 			slog.Info("agent: backup job complete", "job_id", p.JobID)
 		}()
+	case api.JobInit:
+		slog.Info("agent: accepting init job", "job_id", p.JobID)
+		go func() {
+			if err := r.RunInit(ctx, p.JobID); err != nil {
+				slog.Warn("agent: init job failed", "job_id", p.JobID, "err", err)
+				return
+			}
+			slog.Info("agent: init job complete", "job_id", p.JobID)
+		}()
 	default:
 		return fmt.Errorf("kind %q not implemented yet (Phase 2 lands the rest)", p.Kind)
 	}