fix: replace SHA-256 with HKDF + per-row salt for user API key encryption

From the PR description

Summary

• Replaces the single-SHA-256 encryptionKey() with HKDF (RFC 5869) and a per-row random 16-byte salt, eliminating the one-hash-breaks-all vulnerability
• Backward-compatible: rows with salt IS NULL still decrypt via the legacy SHA-256 path; new and updated rows automatically use HKDF
• Adds a salt text column to the user_api_keys table (nullable for compatibility with existing rows)

Closes #67

Changes

• backend/src/lib/userApiKeys.ts - encryptKey/decryptKey exported helpers; deriveKey uses hkdfSync with per-row salt; legacyKey covers null-salt rows
• backend/schema.sql - salt text column added to user_api_keys; run ALTER TABLE public.user_api_keys ADD COLUMN IF NOT EXISTS salt text; on existing databases
• backend/vitest.config.ts + backend/package.json - vitest test runner added
• backend/src/lib/__tests__/userApiKeys.test.ts - 5 unit tests covering HKDF round-trip, unique salts/IVs, tamper detection, and legacy SHA-256 compat

Test plan

• Unit tests: encrypt→decrypt round-trip, unique salt per call, tamper detection, legacy null-salt backward compat
• Build and typecheck pass