feat(conversations): add full-text message search endpoint (M1-006)

Adds GET /api/conversations/search?q=<query> endpoint that searches
message content across all authenticated user's conversations using
PostgreSQL ILIKE. Supports limit/offset pagination (default limit=20)
and returns results sorted by recency with conversation context.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/gateway/src/conversations/conversations.controller.ts

@@ -1,4 +1,5 @@
 import {
+  BadRequestException,
   Body,
   Controller,
   Delete,
@@ -11,6 +12,7 @@ import {
   Param,
   Patch,
   Post,
+  Query,
   UseGuards,
 } from '@nestjs/common';
 import type { Brain } from '@mosaic/brain';
@@ -21,6 +23,7 @@ import {
   CreateConversationDto,
   UpdateConversationDto,
   SendMessageDto,
+  SearchMessagesDto,
 } from './conversations.dto.js';
 
 @Controller('api/conversations')
@@ -33,6 +36,16 @@ export class ConversationsController {
     return this.brain.conversations.findAll(user.id);
   }
 
+  @Get('search')
+  async search(@Query() dto: SearchMessagesDto, @CurrentUser() user: { id: string }) {
+    if (!dto.q || dto.q.trim().length === 0) {
+      throw new BadRequestException('Query parameter "q" is required and must not be empty');
+    }
+    const limit = dto.limit ?? 20;
+    const offset = dto.offset ?? 0;
+    return this.brain.conversations.searchMessages(user.id, dto.q.trim(), limit, offset);
+  }
+
   @Get(':id')
   async findOne(@Param('id') id: string, @CurrentUser() user: { id: string }) {
     const conversation = await this.brain.conversations.findById(id, user.id);

apps/gateway/src/conversations/conversations.dto.ts

@@ -1,12 +1,35 @@
 import {
   IsBoolean,
   IsIn,
+  IsInt,
   IsObject,
   IsOptional,
   IsString,
   IsUUID,
+  Max,
   MaxLength,
+  Min,
 } from 'class-validator';
+import { Type } from 'class-transformer';
+
+export class SearchMessagesDto {
+  @IsString()
+  @MaxLength(500)
+  q!: string;
+
+  @IsOptional()
+  @Type(() => Number)
+  @IsInt()
+  @Min(1)
+  @Max(100)
+  limit?: number = 20;
+
+  @IsOptional()
+  @Type(() => Number)
+  @IsInt()
+  @Min(0)
+  offset?: number = 0;
+}
 
 export class CreateConversationDto {
   @IsOptional()