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>

deploy/install/restic-manager-agent.service

@@ -10,20 +10,34 @@ ExecStart=/usr/local/bin/restic-manager-agent -config /etc/restic-manager/agent.
 Restart=always
 RestartSec=5
 
-# Run as a dedicated unprivileged user; the install script creates it.
-User=restic-manager-agent
-Group=restic-manager-agent
+# The agent runs as root. A fleet-backup tool needs to read every
+# file on the system regardless of DAC permissions; running as a
+# dedicated unprivileged user means either silent skips on /home,
+# /root, /var/lib/<other-daemons>, or operators having to add the
+# service user to every group whose files they want backed up. Both
+# are worse than the threat model already implies (the agent holds
+# repo credentials, executes arbitrary restic, and runs operator-
+# defined hooks — its blast radius is already large).
+#
+# The mitigation is aggressive systemd sandboxing of the root
+# process: drop all capabilities except the few we need, deny
+# writes outside our state dirs, and forbid privilege escalation.
+User=root
+Group=root
 
-# The agent reads its config and writes a small state file there.
-# Anything else is read-only.
-ReadWritePaths=/etc/restic-manager /var/lib/restic-manager
+# CAP_DAC_READ_SEARCH lets us read any file regardless of DAC perms
+# (the "backup everything" capability). CAP_DAC_OVERRIDE is needed
+# during restore for chown/chmod to recreate ownership. Drop the
+# rest — root in this process means "can read", not "can do".
+CapabilityBoundingSet=CAP_DAC_READ_SEARCH CAP_DAC_OVERRIDE CAP_FOWNER CAP_CHOWN
+AmbientCapabilities=CAP_DAC_READ_SEARCH CAP_DAC_OVERRIDE CAP_FOWNER CAP_CHOWN
 
-# Hardening — restic itself needs filesystem read access to whatever
-# paths it's backing up; we don't lock that down here. But everything
-# else gets the standard systemd sandboxing toggles.
+# Hardening — blocks privilege escalation even from root, and
+# confines writes / network / kernel access to what restic actually
+# needs. Filesystem reads stay open: that's the whole job.
 NoNewPrivileges=true
-PrivateTmp=true
 ProtectSystem=strict
+ReadWritePaths=/etc/restic-manager /var/lib/restic-manager
 ProtectHome=read-only
 ProtectHostname=true
 ProtectKernelTunables=true
@@ -31,12 +45,16 @@ ProtectKernelModules=true
 ProtectKernelLogs=true
 ProtectControlGroups=true
 ProtectClock=true
+PrivateTmp=true
+RestrictAddressFamilies=AF_UNIX AF_INET AF_INET6
 RestrictRealtime=true
 RestrictSUIDSGID=true
 RestrictNamespaces=true
 LockPersonality=true
 MemoryDenyWriteExecute=true
 SystemCallArchitectures=native
+SystemCallFilter=@system-service
+SystemCallFilter=~@privileged @resources @reboot @swap @module @raw-io
 
 [Install]
 WantedBy=multi-user.target