feat: delete-only chat moderation and embed search and resolve

- ChatContext implements golem15.chat's MessageModerator: moderator
  group members who are chat members delete in group conversations
  only, never in DMs
- GET /chat/embeds?type=deadline|class|reading&q= (members only,
  at most 20 rows, escaped ILIKE, q at most 100 characters)
- POST /chat/embeds/resolve {refs} (members only, at most 50 distinct
  refs, 8 KiB body): current data or null per ref

classes/chat_context.go

@@ -10,8 +10,9 @@ import (
 )
 
 var (
-	_ chatcontracts.Context        = (*ChatContext)(nil)
-	_ chancontracts.AccessResolver = (*ChatAccess)(nil)
+	_ chatcontracts.Context          = (*ChatContext)(nil)
+	_ chatcontracts.MessageModerator = (*ChatContext)(nil)
+	_ chancontracts.AccessResolver   = (*ChatAccess)(nil)
 )
 
 // The BM chat context: one context of type bm with id 1, whose members are
@@ -75,7 +76,8 @@ func ChatMemberIDs(ctx context.Context, db *gorm.DB, group string) ([]uint, erro
 
 // ChatContext is the golem15.chat context of the BM chat. Membership is the
 // user group named by group; nobody oversees other people's chats, so a
-// moderator reads no DM and edits no foreign message.
+// moderator reads no DM and edits no foreign message. Moderators delete
+// through CanModerateMessage, in group conversations only.
 type ChatContext struct {
 	db             *gorm.DB
 	group          string
@@ -113,6 +115,25 @@ func (c *ChatContext) CanOverseeChats(context.Context, uint) (bool, error) {
 	return false, nil
 }
 
+// CanModerateMessage is golem15.chat's delete-only moderation hook (D-10):
+// a member of the moderator group who is also a chat-group member may
+// delete other people's messages in group conversations (# ogólny). In a
+// DM the answer is always false, so moderators never act on private
+// conversations (D-24). It grants no viewing or editing.
+func (c *ChatContext) CanModerateMessage(ctx context.Context, userID uint, conversationType string) (bool, error) {
+	if conversationType != "group" || userID == 0 || c.moderatorGroup == "" {
+		return false, nil
+	}
+	member, err := c.HasMember(ctx, userID)
+	if err != nil || !member {
+		return false, err
+	}
+	if c.isMember != nil {
+		return c.isMember(ctx, c.db, userID, c.moderatorGroup)
+	}
+	return IsChatModerator(ctx, c.db, userID, c.moderatorGroup)
+}
+
 // HasMember reports whether userID is in the chat group.
 func (c *ChatContext) HasMember(ctx context.Context, userID uint) (bool, error) {
 	if userID == 0 || c.group == "" {