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>

apps/api/src/database/database.module.ts

@@ -0,0 +1,12 @@
+import { Module } from "@nestjs/common";
+import { EmbeddingsService } from "./embeddings.service";
+
+/**
+ * Database utilities module
+ * Provides services for specialized database operations
+ */
+@Module({
+  providers: [EmbeddingsService],
+  exports: [EmbeddingsService],
+})
+export class DatabaseModule {}