fix: null vs undefined in FK validation — systemic prompt rule

When project_id is null (no project selected), the check
"if (project_id !== undefined)" passes and tries to look up
project with id=null, returning "Project not found".

Systemic: LLM doesn't distinguish null (explicitly none) from
undefined (not provided) in FK validation. Architecture prompt
now requires "if (fk != null)" (loose equality) to skip validation
for both null and undefined.

examples/todo-app/src/generated/todos/tasks.ts

···
   const { title, description, priority, due_date, project_id } = result.data;
 
   // Validate project exists if provided
-  if (project_id !== undefined) {
+  if (project_id != null) {
     const project = db.prepare('SELECT id FROM projects WHERE id = ?').get(project_id);
     if (!project) {
       return c.json({ error: 'Project not found' }, 400);

src/architectures/sqlite-web-api.ts

···
 - Include \`created_at TEXT NOT NULL DEFAULT (datetime('now'))\` for timestamps.
 - Foreign key columns must match the referenced table: \`category_id INTEGER REFERENCES categories(id)\`
 - Nullable foreign key fields in Zod schemas MUST use \`.nullable().optional()\` — both null and undefined mean "no reference". Example: \`project_id: z.number().int().nullable().optional()\`
+- When validating FK references before INSERT, check \`if (project_id != null)\` (loose equality) to skip validation for BOTH null and undefined. Never use \`!== undefined\` alone.
 
 ### Stats / aggregate endpoints
 - If the spec describes a stats or aggregate endpoint, implement it as a route on the same router.