feat: add admin role management and user deletion

Admins can now change user roles and permanently delete user accounts.

- New SetRole store method with validation (user/admin only)
- New Delete store method — cascades via foreign keys to sessions,
  faves, and fave_tags
- handleAdminSetRole: change role with self-modification prevention
- handleAdminDeleteUser: permanent deletion with image cleanup from
  disk before cascade delete, self-deletion prevention
- admin_users.html: role dropdown with save button per user row,
  delete button with hx-confirm for safety
- Routes: POST /admin/users/{id}/role, POST /admin/users/{id}/delete

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/handler/admin.go

@@ -11,6 +11,7 @@ import (
 	"strconv"
 	"strings"
 
+	"kode.naiv.no/olemd/favoritter/internal/image"
 	"kode.naiv.no/olemd/favoritter/internal/middleware"
 	"kode.naiv.no/olemd/favoritter/internal/render"
 	"kode.naiv.no/olemd/favoritter/internal/store"
@@ -158,6 +159,88 @@ func (h *Handler) handleAdminToggleDisabled(w http.ResponseWriter, r *http.Reque
 	h.adminUsersFlash(w, r, "Bruker "+user.Username+" er "+action+".", "success")
 }
 
+// handleAdminSetRole changes a user's role.
+func (h *Handler) handleAdminSetRole(w http.ResponseWriter, r *http.Request) {
+	admin := middleware.UserFromContext(r.Context())
+
+	id, err := strconv.ParseInt(r.PathValue("id"), 10, 64)
+	if err != nil {
+		http.NotFound(w, r)
+		return
+	}
+
+	if id == admin.ID {
+		h.adminUsersFlash(w, r, "Du kan ikke endre din egen rolle.", "error")
+		return
+	}
+
+	if err := r.ParseForm(); err != nil {
+		http.Error(w, "Bad request", http.StatusBadRequest)
+		return
+	}
+
+	role := r.FormValue("role")
+	if role != "user" && role != "admin" {
+		h.adminUsersFlash(w, r, "Ugyldig rolle.", "error")
+		return
+	}
+
+	if err := h.deps.Users.SetRole(id, role); err != nil {
+		slog.Error("set role error", "error", err)
+		h.adminUsersFlash(w, r, "Noe gikk galt.", "error")
+		return
+	}
+
+	user, _ := h.deps.Users.GetByID(id)
+	h.adminUsersFlash(w, r, "Rollen til "+user.Username+" er endret til "+role+".", "success")
+}
+
+// handleAdminDeleteUser permanently deletes a user and all their data.
+func (h *Handler) handleAdminDeleteUser(w http.ResponseWriter, r *http.Request) {
+	admin := middleware.UserFromContext(r.Context())
+
+	id, err := strconv.ParseInt(r.PathValue("id"), 10, 64)
+	if err != nil {
+		http.NotFound(w, r)
+		return
+	}
+
+	if id == admin.ID {
+		h.adminUsersFlash(w, r, "Du kan ikke slette din egen konto.", "error")
+		return
+	}
+
+	user, err := h.deps.Users.GetByID(id)
+	if err != nil {
+		slog.Error("get user error", "error", err)
+		h.adminUsersFlash(w, r, "Noe gikk galt.", "error")
+		return
+	}
+
+	// Delete user's images from disk before database deletion.
+	faves, _, _ := h.deps.Faves.ListByUser(id, 100000, 0)
+	for _, f := range faves {
+		if f.ImagePath != "" {
+			if delErr := image.Delete(h.deps.Config.UploadDir, f.ImagePath); delErr != nil {
+				slog.Error("image delete error", "fave_id", f.ID, "error", delErr)
+			}
+		}
+	}
+	if user.AvatarPath != "" {
+		if delErr := image.Delete(h.deps.Config.UploadDir, user.AvatarPath); delErr != nil {
+			slog.Error("avatar delete error", "user_id", id, "error", delErr)
+		}
+	}
+
+	if err := h.deps.Users.Delete(id); err != nil {
+		slog.Error("delete user error", "error", err)
+		h.adminUsersFlash(w, r, "Noe gikk galt.", "error")
+		return
+	}
+
+	h.adminUsersFlash(w, r, "Bruker "+user.Username+" er permanent slettet.", "success")
+}
+
 // handleAdminTags lists all tags.
 func (h *Handler) handleAdminTags(w http.ResponseWriter, r *http.Request) {
 	tags, err := h.deps.Tags.ListAll()