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>

packages/shared/src/constants.ts

@@ -0,0 +1,9 @@
+/**
+ * Shared constants across the monorepo
+ */
+
+/**
+ * Embedding vector dimension for semantic memory
+ * Default: 1536 (OpenAI text-embedding-ada-002 dimension)
+ */
+export const EMBEDDING_DIMENSION = 1536;