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>

internal/server/ws/handler.go

@@ -196,6 +196,16 @@ func dispatchAgentMessage(ctx context.Context, c *Conn, hostID string, env api.E
 			string(p.Status), p.ExitCode, p.Stats, errMsg, p.FinishedAt); err != nil {
 			slog.Warn("ws: mark job finished", "job_id", p.JobID, "err", err)
 		}
+		// A successful backup or init proves the repo exists; flip
+		// repo_initialised_at on the host (idempotent — set-if-null).
+		if p.Status == api.JobSucceeded {
+			if job, err := deps.Store.GetJob(ctx, p.JobID); err == nil &&
+				(job.Kind == string(api.JobBackup) || job.Kind == string(api.JobInit)) {
+				if _, err := deps.Store.MarkHostRepoInitialised(ctx, hostID, p.FinishedAt); err != nil {
+					slog.Warn("ws: mark repo initialised", "host_id", hostID, "err", err)
+				}
+			}
+		}
 		if deps.JobHub != nil {
 			deps.JobHub.Broadcast(p.JobID, env)
 		}
@@ -235,6 +245,15 @@ func dispatchAgentMessage(ctx context.Context, c *Conn, hostID string, env api.E
 		} else {
 			slog.Info("ws: snapshots refreshed", "host_id", hostID, "count", len(snaps))
 		}
+		// A non-empty snapshot list also proves the repo is initialised
+		// (catches the case where an external job — `restic init` from
+		// the CLI, or a backup ran outside this control plane —
+		// initialised it before our first job dispatched).
+		if len(snaps) > 0 {
+			if _, err := deps.Store.MarkHostRepoInitialised(ctx, hostID, time.Now().UTC()); err != nil {
+				slog.Warn("ws: mark repo initialised (snapshots)", "host_id", hostID, "err", err)
+			}
+		}
 
 	case api.MsgRepoStats, api.MsgScheduleAck, api.MsgCommandResult:
 		// TODO(P2): persist these projections.