feat(auth): add login-attempts DB layer and lockout service

Persists per-username failed-attempt counts and computed locked_until
timestamps. Lockout service computes exponential-backoff durations
(min(base * 2^(count-threshold), max)) with auto-unlock once locked_until
passes. Successful login deletes the row, resetting the counter.

Pure DB-keyed lockout — survives server restarts and shares state across
both login routes (HTML and JSON) when wired in a later step.

tests/unit/lockout-service.test.ts

@@ -0,0 +1,127 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import type Database from 'better-sqlite3';
+import { initDb } from '../../src/db/schema.ts';
+import type { Config } from '../../src/config.ts';
+import { createLockoutService } from '../../src/services/lockout.ts';
+import { recordFailure as dbRecordFailure } from '../../src/db/login-attempts.ts';
+
+function makeConfig(overrides: Partial<Config> = {}): Config {
+  return {
+    port: 0,
+    host: '127.0.0.1',
+    jwtSecret: 'x',
+    jwtExpiry: '1h',
+    dbPath: ':memory:',
+    uploadDir: '/tmp',
+    logFile: '/tmp/x.log',
+    maxFileSize: 0,
+    baseUrl: '',
+    cookieSecure: false,
+    trustProxy: false,
+    lockoutThreshold: 3,
+    lockoutBaseSeconds: 10,
+    lockoutMaxSeconds: 80,
+    loginMinResponseMs: 0,
+    loginRateLimitMax: 0,
+    loginRateLimitWindowSeconds: 0,
+    ...overrides,
+  };
+}
+
+describe('lockout service', () => {
+  let db: Database.Database;
+  let nowMs: number;
+  const now = (): Date => new Date(nowMs);
+
+  beforeEach(() => {
+    db = initDb(':memory:');
+    nowMs = Date.UTC(2026, 0, 1, 0, 0, 0);
+  });
+
+  describe('check', () => {
+    it('returns not-locked when no row exists', () => {
+      const svc = createLockoutService({ db, config: makeConfig(), now });
+      expect(svc.check('alice')).toEqual({ locked: false });
+    });
+
+    it('returns not-locked when locked_until is in the past', () => {
+      const past = new Date(nowMs - 1000).toISOString();
+      dbRecordFailure(db, 'alice', past);
+      const svc = createLockoutService({ db, config: makeConfig(), now });
+      expect(svc.check('alice')).toEqual({ locked: false });
+    });
+
+    it('returns locked with retry-after seconds when locked_until is in the future', () => {
+      const future = new Date(nowMs + 30_000).toISOString();
+      dbRecordFailure(db, 'alice', future);
+      const svc = createLockoutService({ db, config: makeConfig(), now });
+      expect(svc.check('alice')).toEqual({ locked: true, retryAfterSeconds: 30 });
+    });
+
+    it('rounds up sub-second remainder so retry-after is never 0', () => {
+      const future = new Date(nowMs + 100).toISOString();
+      dbRecordFailure(db, 'alice', future);
+      const svc = createLockoutService({ db, config: makeConfig(), now });
+      expect(svc.check('alice').retryAfterSeconds).toBe(1);
+    });
+  });
+
+  describe('recordFailure', () => {
+    it('does not lock under threshold', () => {
+      const svc = createLockoutService({ db, config: makeConfig(), now });
+      const r1 = svc.recordFailure('alice');
+      expect(r1).toEqual({ locked: false, durationSeconds: 0, failedCount: 1 });
+      const r2 = svc.recordFailure('alice');
+      expect(r2).toEqual({ locked: false, durationSeconds: 0, failedCount: 2 });
+    });
+
+    it('locks with base duration at threshold', () => {
+      const svc = createLockoutService({ db, config: makeConfig(), now });
+      svc.recordFailure('alice');
+      svc.recordFailure('alice');
+      const r3 = svc.recordFailure('alice');
+      expect(r3.locked).toBe(true);
+      expect(r3.durationSeconds).toBe(10);
+      expect(r3.failedCount).toBe(3);
+    });
+
+    it('doubles duration past threshold', () => {
+      const svc = createLockoutService({ db, config: makeConfig(), now });
+      svc.recordFailure('alice'); // 1
+      svc.recordFailure('alice'); // 2
+      expect(svc.recordFailure('alice').durationSeconds).toBe(10); // 3 -> base
+      expect(svc.recordFailure('alice').durationSeconds).toBe(20); // 4
+      expect(svc.recordFailure('alice').durationSeconds).toBe(40); // 5
+      expect(svc.recordFailure('alice').durationSeconds).toBe(80); // 6 -> cap
+      expect(svc.recordFailure('alice').durationSeconds).toBe(80); // 7 -> still cap
+    });
+
+    it('persists locked_until reachable via check', () => {
+      const svc = createLockoutService({ db, config: makeConfig(), now });
+      svc.recordFailure('alice');
+      svc.recordFailure('alice');
+      svc.recordFailure('alice');
+      const status = svc.check('alice');
+      expect(status.locked).toBe(true);
+      expect(status.retryAfterSeconds).toBe(10);
+    });
+  });
+
+  describe('recordSuccess', () => {
+    it('clears the attempt row', () => {
+      const svc = createLockoutService({ db, config: makeConfig(), now });
+      svc.recordFailure('alice');
+      svc.recordFailure('alice');
+      svc.recordSuccess('alice');
+      // Next failure starts at 1, no lock
+      const r = svc.recordFailure('alice');
+      expect(r.failedCount).toBe(1);
+      expect(r.locked).toBe(false);
+    });
+
+    it('is a no-op for unknown username', () => {
+      const svc = createLockoutService({ db, config: makeConfig(), now });
+      expect(() => svc.recordSuccess('ghost')).not.toThrow();
+    });
+  });
+});