feat(01-02): create Backend Schema Registry with rclone-compatible field definitions

- BackendType union: 'azureblob' | 's3' | 's3-compatible'
- FieldDef interface with key (snake_case rclone key), label, inputType, required, and optional fields
- BACKEND_REGISTRY: azureblob (3 fields), s3 (4 fields), s3-compatible (5 fields)
- All FieldDef.key values match rclone INI config key names exactly
- registry.test.ts: 7 tests all passing (GREEN)

src/schemas/registry.test.ts

@@ -0,0 +1,55 @@
+import { describe, it, expect } from 'vitest';
+import { BACKEND_REGISTRY, BackendType } from './registry';
+
+const EXPECTED_BACKENDS: BackendType[] = ['azureblob', 's3', 's3-compatible'];
+
+describe('Backend Schema Registry', () => {
+  it('exports all three required backend types', () => {
+    for (const backend of EXPECTED_BACKENDS) {
+      expect(BACKEND_REGISTRY[backend]).toBeDefined();
+    }
+  });
+
+  it('each backend has at least one field definition', () => {
+    for (const backend of EXPECTED_BACKENDS) {
+      expect(BACKEND_REGISTRY[backend].length).toBeGreaterThan(0);
+    }
+  });
+
+  it('each FieldDef has a non-empty key (snake_case, no camelCase)', () => {
+    for (const backend of EXPECTED_BACKENDS) {
+      for (const field of BACKEND_REGISTRY[backend]) {
+        expect(field.key).toBeTruthy();
+        // Reject camelCase — rclone keys are snake_case or lowercase
+        expect(field.key).not.toMatch(/[A-Z]/);
+      }
+    }
+  });
+
+  it('each FieldDef has a non-empty label', () => {
+    for (const backend of EXPECTED_BACKENDS) {
+      for (const field of BACKEND_REGISTRY[backend]) {
+        expect(field.label).toBeTruthy();
+      }
+    }
+  });
+
+  it('Azure Blob has account field (required)', () => {
+    const accountField = BACKEND_REGISTRY.azureblob.find(f => f.key === 'account');
+    expect(accountField).toBeDefined();
+    expect(accountField!.required).toBe(true);
+  });
+
+  it('S3 has access_key_id, secret_access_key, and region fields', () => {
+    const keys = BACKEND_REGISTRY.s3.map(f => f.key);
+    expect(keys).toContain('access_key_id');
+    expect(keys).toContain('secret_access_key');
+    expect(keys).toContain('region');
+  });
+
+  it('S3-compatible has endpoint field (required)', () => {
+    const endpointField = BACKEND_REGISTRY['s3-compatible'].find(f => f.key === 'endpoint');
+    expect(endpointField).toBeDefined();
+    expect(endpointField!.required).toBe(true);
+  });
+});