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/package.json

@@ -15,13 +15,23 @@
     "test": "vitest run",
     "test:watch": "vitest",
     "test:coverage": "vitest run --coverage",
-    "test:e2e": "vitest run --config ./vitest.e2e.config.ts"
+    "test:e2e": "vitest run --config ./vitest.e2e.config.ts",
+    "prisma:generate": "prisma generate",
+    "prisma:migrate": "prisma migrate dev",
+    "prisma:migrate:prod": "prisma migrate deploy",
+    "prisma:studio": "prisma studio",
+    "prisma:seed": "prisma db seed",
+    "prisma:reset": "prisma migrate reset"
+  },
+  "prisma": {
+    "seed": "tsx prisma/seed.ts"
   },
   "dependencies": {
+    "@mosaic/shared": "workspace:*",
     "@nestjs/common": "^11.1.12",
     "@nestjs/core": "^11.1.12",
     "@nestjs/platform-express": "^11.1.12",
-    "@mosaic/shared": "workspace:*",
+    "@prisma/client": "^6.19.2",
     "reflect-metadata": "^0.2.2",
     "rxjs": "^7.8.1"
   },
@@ -33,6 +43,8 @@
     "@swc/core": "^1.10.18",
     "@types/express": "^5.0.1",
     "@types/node": "^22.13.4",
+    "prisma": "^6.19.2",
+    "tsx": "^4.21.0",
     "typescript": "^5.8.2",
     "unplugin-swc": "^1.5.2",
     "vitest": "^3.0.8"