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>

.env.example

@@ -5,8 +5,16 @@ API_HOST=0.0.0.0
 # Web Configuration
 NEXT_PUBLIC_API_URL=http://localhost:3001
 
-# Database (configured in later milestone)
-# DATABASE_URL=postgresql://user:password@localhost:5432/mosaic
+# Database
+DATABASE_URL=postgresql://mosaic:mosaic_dev_password@localhost:5432/mosaic
+POSTGRES_USER=mosaic
+POSTGRES_PASSWORD=mosaic_dev_password
+POSTGRES_DB=mosaic
+POSTGRES_PORT=5432
+
+# Valkey (Redis-compatible cache)
+VALKEY_URL=redis://localhost:6379
+VALKEY_PORT=6379
 
 # Authentication (configured in later milestone)
 # OIDC_ISSUER=https://auth.example.com