Add public feed, admin area, self-registration, visibility & hashtags

- Public feed (/) with infinite scroll via Intersection Observer
- Self-registration (/register)
- Admin area (/admin/entries, /admin/users) with user management
- journal_entries: visibility (public/private) + hashtags fields
- users: is_admin flag
- DB schema updated (recreate DB to apply)
- CI: run go test via docker run (golang:1.25-alpine) — fixes 'go not found'

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/api/journal.go

@@ -46,6 +46,20 @@ func (h *JournalHandler) HandleCreateEntry(w http.ResponseWriter, r *http.Reques
 	entryTime := strings.TrimSpace(r.FormValue("time"))
 	title := strings.TrimSpace(r.FormValue("title"))
 	description := strings.TrimSpace(r.FormValue("description"))
+	visibility := r.FormValue("visibility")
+	if visibility != "public" && visibility != "private" {
+		visibility = "private"
+	}
+	var hashtags []string
+	if raw := strings.TrimSpace(r.FormValue("hashtags")); raw != "" {
+		for _, tag := range strings.Split(raw, ",") {
+			tag = strings.TrimSpace(tag)
+			tag = strings.TrimPrefix(tag, "#")
+			if tag != "" {
+				hashtags = append(hashtags, tag)
+			}
+		}
+	}
 
 	if date == "" || entryTime == "" {
 		http.Error(w, "Datum und Uhrzeit sind Pflichtfelder", http.StatusBadRequest)
@@ -58,6 +72,8 @@ func (h *JournalHandler) HandleCreateEntry(w http.ResponseWriter, r *http.Reques
 		EntryTime:   entryTime,
 		Title:       title,
 		Description: description,
+		Visibility:  visibility,
+		Hashtags:    hashtags,
 	}
 
 	if lat := r.FormValue("lat"); lat != "" {