feat(#379): Register MatrixService in BridgeModule with conditional loading

- Add CHAT_PROVIDERS injection token for bridge-agnostic access
- Conditional loading based on env vars (DISCORD_BOT_TOKEN, MATRIX_ACCESS_TOKEN)
- Both bridges can run simultaneously
- No crash if neither bridge is configured
- Tests verify all configuration combinations

Refs #379

apps/api/src/bridge/bridge.module.ts

@@ -1,16 +1,55 @@
-import { Module } from "@nestjs/common";
+import { Logger, Module } from "@nestjs/common";
 import { DiscordService } from "./discord/discord.service";
+import { MatrixService } from "./matrix/matrix.service";
 import { StitcherModule } from "../stitcher/stitcher.module";
+import { CHAT_PROVIDERS } from "./bridge.constants";
+import type { IChatProvider } from "./interfaces";
+
+const logger = new Logger("BridgeModule");
 
 /**
  * Bridge Module - Chat platform integrations
  *
- * Provides integration with chat platforms (Discord, Slack, Matrix, etc.)
+ * Provides integration with chat platforms (Discord, Matrix, etc.)
  * for controlling Mosaic Stack via chat commands.
+ *
+ * Both services are always registered as providers, but the CHAT_PROVIDERS
+ * injection token only includes bridges whose environment variables are set:
+ * - Discord: included when DISCORD_BOT_TOKEN is set
+ * - Matrix: included when MATRIX_ACCESS_TOKEN is set
+ *
+ * Both bridges can run simultaneously, and no error occurs if neither is configured.
+ * Consumers should inject CHAT_PROVIDERS for bridge-agnostic access to all active providers.
  */
 @Module({
   imports: [StitcherModule],
-  providers: [DiscordService],
-  exports: [DiscordService],
+  providers: [
+    DiscordService,
+    MatrixService,
+    {
+      provide: CHAT_PROVIDERS,
+      useFactory: (discord: DiscordService, matrix: MatrixService): IChatProvider[] => {
+        const providers: IChatProvider[] = [];
+
+        if (process.env.DISCORD_BOT_TOKEN) {
+          providers.push(discord);
+          logger.log("Discord bridge enabled (DISCORD_BOT_TOKEN detected)");
+        }
+
+        if (process.env.MATRIX_ACCESS_TOKEN) {
+          providers.push(matrix);
+          logger.log("Matrix bridge enabled (MATRIX_ACCESS_TOKEN detected)");
+        }
+
+        if (providers.length === 0) {
+          logger.warn("No chat bridges configured. Set DISCORD_BOT_TOKEN or MATRIX_ACCESS_TOKEN.");
+        }
+
+        return providers;
+      },
+      inject: [DiscordService, MatrixService],
+    },
+  ],
+  exports: [DiscordService, MatrixService, CHAT_PROVIDERS],
 })
 export class BridgeModule {}