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/domain/models.go

@@ -56,6 +56,8 @@ type JournalEntry struct {
 	Description string         `json:"description"`
 	Lat         *float64       `json:"lat,omitempty"`
 	Lon         *float64       `json:"lon,omitempty"`
+	Visibility  string         `json:"visibility"` // "public" | "private"
+	Hashtags    []string       `json:"hashtags"`
 	CreatedAt   time.Time      `json:"created_at"`
 	Images      []JournalImage `json:"images,omitempty"`
 }
@@ -74,6 +76,7 @@ type User struct {
 	UserID       string    `json:"user_id"`
 	Username     string    `json:"username"`
 	PasswordHash string    `json:"-"`
+	IsAdmin      bool      `json:"is_admin"`
 	CreatedAt    time.Time `json:"created_at"`
 }