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.dev.yml

@@ -0,0 +1,22 @@
+# Development overrides for docker-compose.yml
+# Usage: docker compose -f docker-compose.yml -f docker-compose.dev.yml up
+
+services:
+  postgres:
+    environment:
+      POSTGRES_USER: mosaic
+      POSTGRES_PASSWORD: mosaic_dev_password
+      POSTGRES_DB: mosaic
+    ports:
+      - "5432:5432"
+    # Enable query logging for development
+    command:
+      - "postgres"
+      - "-c"
+      - "log_statement=all"
+      - "-c"
+      - "log_duration=on"
+
+  valkey:
+    ports:
+      - "6379:6379"