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.constants.ts

@@ -0,0 +1,15 @@
+/**
+ * Bridge Module Constants
+ *
+ * Injection tokens for the bridge module.
+ */
+
+/**
+ * Injection token for the array of active IChatProvider instances.
+ *
+ * Use this token to inject all configured chat providers:
+ * ```
+ * @Inject(CHAT_PROVIDERS) private readonly chatProviders: IChatProvider[]
+ * ```
+ */
+export const CHAT_PROVIDERS = "CHAT_PROVIDERS";