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/restic/url.go

@@ -0,0 +1,85 @@
+package restic
+
+import (
+	"net/url"
+	"strings"
+)
+
+// mergeRestCreds embeds basic-auth user:pass into a `rest:` URL, only
+// at the moment we hand it off to the restic subprocess. The result
+// is intentionally NOT stored on Env or logged — restic's REST
+// backend reads basic-auth from the URL only, so we have nowhere
+// else to put them. Callers must treat the return value as
+// secret-bearing and feed it straight into exec env.
+//
+// No-ops when:
+//   - the URL has no `rest:` prefix (other backends — s3, b2, sftp,
+//     etc. — get creds via their own env vars);
+//   - the URL already embeds user:pass (operator typed creds inline);
+//   - username is empty.
+//
+// Returns rawURL unchanged if it can't be parsed; restic will then
+// reject it and the operator gets a clear error rather than a silent
+// "I quietly stripped your URL" surprise.
+func mergeRestCreds(rawURL, username, password string) string {
+	if !strings.HasPrefix(rawURL, "rest:") {
+		return rawURL
+	}
+	if username == "" {
+		return rawURL
+	}
+	inner := strings.TrimPrefix(rawURL, "rest:")
+	u, err := url.Parse(inner)
+	if err != nil || u.Host == "" {
+		// Either unparseable or a relative URL we shouldn't touch —
+		// pass through and let restic complain with a clear message.
+		return rawURL
+	}
+	if u.User != nil {
+		// Operator already embedded creds — don't overwrite.
+		return rawURL
+	}
+	u.User = url.UserPassword(username, password)
+	return "rest:" + u.String()
+}
+
+// RedactURL returns a logging-safe version of u with any password in
+// the userinfo replaced by ***. Mirrors restic's own redaction so
+// our logs match what restic prints. Use this — never the bare URL —
+// whenever a URL might end up in slog output, audit entries, or any
+// surface an operator can read.
+//
+// Non-restic URLs (s3, b2, sftp, …) pass through unchanged unless
+// they happen to embed userinfo, in which case we redact the same
+// way for consistency.
+func RedactURL(u string) string {
+	prefix := ""
+	rest := u
+	if i := strings.Index(u, ":"); i > 0 && i+3 < len(u) && u[i+1:i+3] == "//" {
+		// scheme://… — keep "scheme:" intact.
+		prefix = u[:i+1]
+		rest = u[i+1:]
+	} else if strings.HasPrefix(u, "rest:") {
+		prefix = "rest:"
+		rest = strings.TrimPrefix(u, "rest:")
+	}
+	parsed, err := url.Parse(rest)
+	if err != nil || parsed.User == nil {
+		return u
+	}
+	if _, hasPass := parsed.User.Password(); !hasPass {
+		return u
+	}
+	// Build the redacted form by hand rather than via url.URL.String(),
+	// which percent-encodes the redaction marker into "%2A%2A%2A".
+	user := parsed.User.Username()
+	parsed.User = nil
+	rebuilt := parsed.String()
+	// rebuilt is "scheme://host/path…"; splice user:***@ in after "//".
+	const sep = "//"
+	idx := strings.Index(rebuilt, sep)
+	if idx < 0 {
+		return u
+	}
+	return prefix + rebuilt[:idx+len(sep)] + user + ":***@" + rebuilt[idx+len(sep):]
+}