feat(01-03): implement BACKEND_SCHEMAS Zod schema builder

- buildZodSchema derives z.object() from BACKEND_REGISTRY field definitions
- required fields map to z.string().min(1, label) — optional to z.string().optional()
- BACKEND_SCHEMAS exported with entries for azureblob, s3, s3-compatible
- BackendFormValues<T> utility type infers TypeScript type from schema
- No hardcoded field names — all shapes derived from registry loop
- All 8 tests in index.test.ts pass (GREEN)

src/schemas/index.ts

@@ -0,0 +1,28 @@
+// src/schemas/index.ts
+// Zod schemas derived programmatically from the Backend Schema Registry.
+// DO NOT hand-write z.object() calls with hardcoded field names — all shapes come from the registry.
+// Adding a field to BACKEND_REGISTRY automatically adds it to validation.
+
+import { z } from 'zod';
+import { BACKEND_REGISTRY, BackendType } from './registry';
+
+function buildZodSchema(backendType: BackendType): z.ZodObject<Record<string, z.ZodTypeAny>> {
+  const fields = BACKEND_REGISTRY[backendType];
+  const shape: Record<string, z.ZodTypeAny> = {};
+  for (const field of fields) {
+    shape[field.key] = field.required
+      ? z.string().min(1, `${field.label} is required`)
+      : z.string().optional();
+  }
+  return z.object(shape);
+}
+
+export const BACKEND_SCHEMAS = {
+  azureblob:        buildZodSchema('azureblob'),
+  s3:               buildZodSchema('s3'),
+  's3-compatible':  buildZodSchema('s3-compatible'),
+} as const;
+
+// Utility type: infer TypeScript type from a backend's Zod schema
+export type BackendFormValues<T extends BackendType> =
+  z.infer<typeof BACKEND_SCHEMAS[T]>;