fix(auth): trim username and email on all write paths

Self-registration stored values verbatim, so trailing whitespace could
produce rows that lookup code (which trims input) silently misses.
Trim username and email before validation and INSERT in registerUser,
adminService.updateUser, and oidcService.findOrCreateUser. updateSettings
and adminService.createUser already trimmed correctly.

Adds a one-shot backfill migration (trimUserWhitespace) that trims
existing dirty rows; collisions are resolved by appending __migrated_<id>
to the value with a loud console.warn so operators can review affected
accounts.

18 new tests covering registration trim, duplicate detection, admin
update trim, trip-member lookup regression, and all migration branches.

server/tests/integration/admin.test.ts

@@ -368,6 +368,53 @@ describe('Admin user management', () => {
   });
 });
 
+// ─────────────────────────────────────────────────────────────────────────────
+// Admin user management — whitespace normalization
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe('Admin user management — whitespace normalization', () => {
+  it('ADMIN-UPDATE-TRIM-1 — PUT /admin/users/:id trims username before storing', async () => {
+    const { user: admin } = createAdmin(testDb);
+    const { user } = createUser(testDb);
+
+    const res = await request(app)
+      .put(`/api/admin/users/${user.id}`)
+      .set('Cookie', authCookie(admin.id))
+      .send({ username: '  trimmedadmin  ' });
+
+    expect(res.status).toBe(200);
+    const row = testDb.prepare('SELECT username FROM users WHERE id = ?').get(user.id) as { username: string };
+    expect(row.username).toBe('trimmedadmin');
+  });
+
+  it('ADMIN-UPDATE-TRIM-2 — PUT /admin/users/:id trims email before storing', async () => {
+    const { user: admin } = createAdmin(testDb);
+    const { user } = createUser(testDb);
+
+    const res = await request(app)
+      .put(`/api/admin/users/${user.id}`)
+      .set('Cookie', authCookie(admin.id))
+      .send({ email: '  newemail@example.com  ' });
+
+    expect(res.status).toBe(200);
+    const row = testDb.prepare('SELECT email FROM users WHERE id = ?').get(user.id) as { email: string };
+    expect(row.email).toBe('newemail@example.com');
+  });
+
+  it('ADMIN-UPDATE-TRIM-3 — PUT /admin/users/:id with whitespace-padded username that trims to existing returns 409', async () => {
+    const { user: admin } = createAdmin(testDb);
+    const { user: existing } = createUser(testDb, { username: 'carol' });
+    const { user: target } = createUser(testDb);
+
+    const res = await request(app)
+      .put(`/api/admin/users/${target.id}`)
+      .set('Cookie', authCookie(admin.id))
+      .send({ username: `  ${existing.username}  ` });
+
+    expect(res.status).toBe(409);
+  });
+});
+
 // ─────────────────────────────────────────────────────────────────────────────
 // System stats
 // ─────────────────────────────────────────────────────────────────────────────