fix(security): close Critical IDOR/authz gaps (C-1/C-2)

C-1: messaging endpoints now enforce channel participation (public
     channels open; private require channel_members). authorUserId is
     forced to the authenticated user (no more author spoofing); edit/
     delete require message-author ownership; history read gated too.
C-2: PUT /commands body strictly validated + size-capped via
     SyncCommandsDto (kills catalog poisoning / DoS). Optional
     FABRIC_BACKEND_GUILD_COMMANDS_SYNC_KEY restricts the write to the
     plugin when set; never weaker than before when unset.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/commands/dto.sync-commands.dto.ts

@@ -0,0 +1,102 @@
+import {
+  ArrayMaxSize,
+  IsArray,
+  IsBoolean,
+  IsOptional,
+  IsString,
+  MaxLength,
+  ValidateNested,
+} from 'class-validator';
+import { Type } from 'class-transformer';
+
+// Guild C-2: the slash-command catalog is guild-global and rendered by the
+// frontend `/` autocomplete. Without a strict schema + caps a single
+// authenticated caller could poison it or blow up the DB / clients.
+// The global ValidationPipe runs with { whitelist, forbidNonWhitelisted },
+// so any unknown field is rejected.
+
+class CommandChoiceDto {
+  @IsString()
+  @MaxLength(200)
+  value!: string;
+
+  @IsString()
+  @MaxLength(200)
+  label!: string;
+}
+
+class CommandArgDto {
+  @IsString()
+  @MaxLength(100)
+  name!: string;
+
+  @IsOptional()
+  @IsString()
+  @MaxLength(500)
+  description?: string;
+
+  @IsOptional()
+  @IsString()
+  @MaxLength(40)
+  type?: string;
+
+  @IsOptional()
+  @IsBoolean()
+  required?: boolean;
+
+  @IsOptional()
+  @IsBoolean()
+  captureRemaining?: boolean;
+
+  @IsOptional()
+  @IsBoolean()
+  preferAutocomplete?: boolean;
+
+  // null when there are no choices (plugin sends explicit null).
+  @IsOptional()
+  @IsArray()
+  @ArrayMaxSize(100)
+  @ValidateNested({ each: true })
+  @Type(() => CommandChoiceDto)
+  choices?: CommandChoiceDto[] | null;
+}
+
+class CommandSpecDto {
+  @IsString()
+  @MaxLength(100)
+  name!: string;
+
+  @IsOptional()
+  @IsString()
+  @MaxLength(100)
+  nativeName?: string;
+
+  @IsOptional()
+  @IsString()
+  @MaxLength(500)
+  description?: string;
+
+  @IsOptional()
+  @IsBoolean()
+  acceptsArgs?: boolean;
+
+  @IsOptional()
+  @IsArray()
+  @ArrayMaxSize(50)
+  @ValidateNested({ each: true })
+  @Type(() => CommandArgDto)
+  args?: CommandArgDto[];
+
+  @IsOptional()
+  @IsString()
+  @MaxLength(20)
+  argsParsing?: string;
+}
+
+export class SyncCommandsDto {
+  @IsArray()
+  @ArrayMaxSize(200)
+  @ValidateNested({ each: true })
+  @Type(() => CommandSpecDto)
+  commands!: CommandSpecDto[];
+}