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/app.module.ts

@@ -1,9 +1,11 @@
 import { Module } from "@nestjs/common";
 import { AppController } from "./app.controller";
 import { AppService } from "./app.service";
+import { PrismaModule } from "./prisma/prisma.module";
+import { DatabaseModule } from "./database/database.module";
 
 @Module({
-  imports: [],
+  imports: [PrismaModule, DatabaseModule],
   controllers: [AppController],
   providers: [AppService],
 })