feat: implement Story 1.1 — create and view diagrams

Add diagram/project DB schema, CRUD API, dashboard pages with grid/card/
empty state, create dialog with type selector, editor placeholder, and
26 schema validation tests. Includes code review fixes: soft-delete
filter on GET /:id, error handling, keyboard accessibility, type-safe
API response types, and error states on pages.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/db/src/schema/diagram.ts

@@ -0,0 +1,71 @@
+import {
+  integer,
+  jsonb,
+  pgEnum,
+  pgTable,
+  text,
+  timestamp,
+} from "drizzle-orm/pg-core";
+
+import { generateId } from "@turbostarter/shared/utils";
+
+import {
+  createInsertSchema,
+  createSelectSchema,
+  createUpdateSchema,
+} from "../lib/zod";
+
+import { user } from "./auth";
+
+import type * as z from "zod";
+
+export const diagramTypeEnum = pgEnum("diagram_type", [
+  "bpmn",
+  "er",
+  "orgchart",
+  "architecture",
+  "sequence",
+  "flowchart",
+]);
+
+export const project = pgTable("project", {
+  id: text().primaryKey().notNull().$defaultFn(generateId),
+  name: text().notNull(),
+  userId: text()
+    .references(() => user.id, { onDelete: "cascade" })
+    .notNull(),
+  sortOrder: integer().default(0),
+  createdAt: timestamp().defaultNow(),
+  updatedAt: timestamp().$onUpdate(() => new Date()),
+});
+
+export const diagram = pgTable("diagram", {
+  id: text().primaryKey().notNull().$defaultFn(generateId),
+  title: text().notNull(),
+  type: diagramTypeEnum().notNull(),
+  graphData: jsonb().$type<object>().default({}),
+  userId: text()
+    .references(() => user.id, { onDelete: "cascade" })
+    .notNull(),
+  projectId: text(),
+  lastAiMessage: text(),
+  deletedAt: timestamp(),
+  createdAt: timestamp().defaultNow(),
+  updatedAt: timestamp().$onUpdate(() => new Date()),
+});
+
+export const insertDiagramSchema = createInsertSchema(diagram);
+export const selectDiagramSchema = createSelectSchema(diagram);
+export const updateDiagramSchema = createUpdateSchema(diagram);
+
+export type InsertDiagram = z.infer<typeof insertDiagramSchema>;
+export type SelectDiagram = z.infer<typeof selectDiagramSchema>;
+export type UpdateDiagram = z.infer<typeof updateDiagramSchema>;
+
+export const insertProjectSchema = createInsertSchema(project);
+export const selectProjectSchema = createSelectSchema(project);
+export const updateProjectSchema = createUpdateSchema(project);
+
+export type InsertProject = z.infer<typeof insertProjectSchema>;
+export type SelectProject = z.infer<typeof selectProjectSchema>;
+export type UpdateProject = z.infer<typeof updateProjectSchema>;