migrate: replace pg with Prisma ORM

- Add prisma/schema.prisma with Project/Task models, enums, and relations
- Create src/lib/db.ts singleton Prisma client
- Refactor all 5 API routes to use Prisma queries
- Replace migrate.ts with seed.ts for initial data
- Update Dockerfile for Prisma lifecycle (copy generated client)
- Update tsconfig.json with @/generated/* path alias
- Remove pg and @types/pg dependencies
- Add prisma.config.ts for Prisma 6 config
- Update .gitignore for generated Prisma client

prisma/schema.prisma

@@ -0,0 +1,76 @@
+// This is your Prisma schema file,
+// learn more about it in the docs: https://pris.ly/d/prisma-schema
+
+generator client {
+  provider = "prisma-client"
+  output   = "../src/generated/prisma"
+}
+
+datasource db {
+  provider = "postgresql"
+  url      = env("DATABASE_URL")
+}
+
+model Project {
+  id          String   @id @default(uuid()) @db.Uuid
+  name        String
+  description String   @default("")
+  color       String   @default("#3b82f6")
+  sortOrder   Int      @default(0) @map("sort_order")
+  createdAt   DateTime @default(now()) @map("created_at")
+  updatedAt   DateTime @updatedAt @map("updated_at")
+  tasks       Task[]
+
+  @@index([sortOrder])
+}
+
+model Task {
+  id                String       @id @default(uuid()) @db.Uuid
+  title             String
+  description       String       @default("")
+  completed         Boolean      @default(false)
+  priority          Priority     @default(medium)
+  dueDate           DateTime?  @map("due_date")
+  status            Status       @default(todo)
+  parentTaskId      String?      @map("parent_task_id") @db.Uuid
+  recurrenceRule    Recurrence   @default(none) @map("recurrence_rule")
+  recurrenceInterval Int         @default(1) @map("recurrence_interval")
+  nextOccurrence    DateTime?    @map("next_occurrence")
+  sortOrder         Int          @default(0) @map("sort_order")
+  createdAt         DateTime     @default(now()) @map("created_at")
+  updatedAt         DateTime     @updatedAt @map("updated_at")
+
+  parentTask   Task?       @relation("TaskChildren", fields: [parentTaskId], references: [id], onDelete: Cascade)
+  children     Task[]      @relation("TaskChildren")
+  project      Project?    @relation(fields: [projectId], references: [id], onDelete: Cascade)
+  projectId    String?     @map("project_id") @db.Uuid
+
+  @@index([projectId])
+  @@index([parentTaskId])
+  @@index([completed])
+  @@index([status])
+  @@index([dueDate])
+  @@index([recurrenceRule])
+}
+
+enum Priority {
+  low
+  medium
+  high
+  urgent
+}
+
+enum Status {
+  todo
+  in_progress
+  done
+}
+
+enum Recurrence {
+  none
+  daily
+  weekly
+  biweekly
+  monthly
+  yearly
+}