feat(#2): Implement PostgreSQL 17 + pgvector database schema

Establishes multi-tenant database layer with vector similarity search for AI-powered memory features. Includes Docker infrastructure, Prisma ORM integration, NestJS services, and shared types across the monorepo.

Key changes:
- Docker: PostgreSQL 17 + pgvector v0.7.4, Valkey cache
- Schema: 8 models (User, Workspace, Task, Event, Project, ActivityLog, MemoryEmbedding) with RLS preparation
- NestJS: PrismaModule, DatabaseModule, EmbeddingsService
- Shared: Type-safe enums, constants, and database types

Fixes #2

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docker/docker-compose.yml

@@ -0,0 +1,51 @@
+services:
+  postgres:
+    build:
+      context: ./postgres
+      dockerfile: Dockerfile
+    container_name: mosaic-postgres
+    restart: unless-stopped
+    environment:
+      POSTGRES_USER: ${POSTGRES_USER:-mosaic}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-mosaic_dev_password}
+      POSTGRES_DB: ${POSTGRES_DB:-mosaic}
+    ports:
+      - "${POSTGRES_PORT:-5432}:5432"
+    volumes:
+      - postgres_data:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-mosaic} -d ${POSTGRES_DB:-mosaic}"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+      start_period: 30s
+    networks:
+      - mosaic-network
+
+  valkey:
+    image: valkey/valkey:8-alpine
+    container_name: mosaic-valkey
+    restart: unless-stopped
+    ports:
+      - "${VALKEY_PORT:-6379}:6379"
+    volumes:
+      - valkey_data:/data
+    healthcheck:
+      test: ["CMD", "valkey-cli", "ping"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+      start_period: 10s
+    networks:
+      - mosaic-network
+
+volumes:
+  postgres_data:
+    name: mosaic-postgres-data
+  valkey_data:
+    name: mosaic-valkey-data
+
+networks:
+  mosaic-network:
+    name: mosaic-network
+    driver: bridge