Phase 7: monitoring UI — send log, queue, mail.log tail

Three HTMX-polled monitoring screens (spec 7.2.11-13): send log with
server-side domain/application filters and pagination, Postfix queue
(postqueue -p), and a mail.log tail. Fragment endpoints return HTML
snippets, not JSON (spec 7.1); all output is auto-escaped via
html/template (spec 7.6.7).

Adds store.QuerySendLog/CountSendLog/ListApplicationLogins,
postfix.Queue(), and logtail.TailLines (a point-in-time reverse read,
independent of the background follow loop). Verified on the dev server:
gofmt/vet/test green, docker build green, container e2e (filters,
60-row pagination, <script> escaping, real postqueue/mail.log output,
existing Reload button unaffected).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/web/templates.go

@@ -7,32 +7,60 @@ import (
 	"net/http"
 )
 
-// templates holds the parsed page templates. Each page is parsed together with
-// the shared base layout so {{ template "base" . }} works. Rendering goes
-// through html/template, which auto-escapes all interpolated data (spec 7.6.7).
+// templates holds the parsed page and fragment templates. Each page is parsed
+// together with the shared base layout so {{ template "base" . }} works.
+// Fragments (HTMX polling targets, spec 7.1) are parsed standalone, without
+// the layout, so they can be swapped into an existing page as an HTML snippet
+// rather than a full document. Rendering always goes through html/template,
+// which auto-escapes all interpolated data regardless (spec 7.6.7).
 type templates struct {
-	pages map[string]*template.Template
+	pages     map[string]*template.Template
+	fragments map[string]*template.Template
 }
 
-// pageFiles maps a logical page name to its template file. Every page composes
-// with layout.html.
-var pageFiles = map[string]string{
-	"setup":         "templates/setup.html",
-	"login":         "templates/login.html",
-	"dashboard":     "templates/dashboard.html",
-	"domain_detail": "templates/domain_detail.html",
-	"domain_delete": "templates/domain_delete.html",
+// pageFiles maps a logical page name to its template files. Every page
+// composes with layout.html; pages that embed a polling fragment (spec 7.1)
+// list that fragment's file too, so the same {{define}} block renders both
+// the initial page and the fragment's own refresh responses identically.
+var pageFiles = map[string][]string{
+	"setup":         {"templates/setup.html"},
+	"login":         {"templates/login.html"},
+	"dashboard":     {"templates/dashboard.html"},
+	"domain_detail": {"templates/domain_detail.html"},
+	"domain_delete": {"templates/domain_delete.html"},
+	"sendlog":       {"templates/sendlog.html", "templates/sendlog_rows.html"},
+	"queue":         {"templates/queue.html", "templates/queue_body.html"},
+	"logtail":       {"templates/logtail.html", "templates/logtail_body.html"},
+}
+
+// fragmentFiles maps a fragment name (also its {{define}} block name) to its
+// template file, for standalone rendering by the HTMX polling endpoints.
+var fragmentFiles = map[string]string{
+	"sendlog_rows": "templates/sendlog_rows.html",
+	"queue_body":   "templates/queue_body.html",
+	"logtail_body": "templates/logtail_body.html",
 }
 
 func loadTemplates() (*templates, error) {
-	t := &templates{pages: make(map[string]*template.Template)}
-	for name, file := range pageFiles {
-		tmpl, err := template.New("layout.html").ParseFS(assetsFS, "templates/layout.html", file)
+	t := &templates{
+		pages:     make(map[string]*template.Template),
+		fragments: make(map[string]*template.Template),
+	}
+	for name, files := range pageFiles {
+		patterns := append([]string{"templates/layout.html"}, files...)
+		tmpl, err := template.New("layout.html").ParseFS(assetsFS, patterns...)
 		if err != nil {
 			return nil, fmt.Errorf("parse template %s: %w", name, err)
 		}
 		t.pages[name] = tmpl
 	}
+	for name, file := range fragmentFiles {
+		tmpl, err := template.ParseFS(assetsFS, file)
+		if err != nil {
+			return nil, fmt.Errorf("parse fragment %s: %w", name, err)
+		}
+		t.fragments[name] = tmpl
+	}
 	return t, nil
 }
 
@@ -54,3 +82,22 @@ func (s *Server) render(w http.ResponseWriter, status int, page string, data any
 	w.WriteHeader(status)
 	_, _ = buf.WriteTo(w)
 }
+
+// renderFragment writes an HTMX polling fragment as a bare HTML snippet, with
+// no surrounding layout (spec 7.1: fragment endpoints return HTML, not JSON).
+func (s *Server) renderFragment(w http.ResponseWriter, status int, name string, data any) {
+	tmpl, ok := s.tmpl.fragments[name]
+	if !ok {
+		http.Error(w, "template not found", http.StatusInternalServerError)
+		return
+	}
+	var buf bytes.Buffer
+	if err := tmpl.ExecuteTemplate(&buf, name, data); err != nil {
+		logf("panel: render fragment %s: %v", name, err)
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	w.WriteHeader(status)
+	_, _ = buf.WriteTo(w)
+}