feat: real SCRAM-SHA-256 authentication (Option C)

Security:
- New scram.nim module with full SCRAM-SHA-256 per RFC 7677
  * PBKDF2-HMAC-SHA-256 key derivation
  * HMAC-SHA-256 with multiple overloads for bytes/strings
  * Secure nonce/salt generation via /dev/urandom
  * Client/server message parsing and proof verification

AuthManager updates:
- registerScramUser(): stores salted credentials (salt + iterations + storedKey + serverKey)
- startScram(): initiates challenge-response handshake
- finishScram(): verifies client proof and returns server signature
- Legacy amSCRAMSHA256 path kept for backward compatibility

HTTP endpoints:
- POST /auth/scram/start — accepts client-first-message, returns server-first
- POST /auth/scram/finish — accepts client-final-message, returns server-final

Tests:
- SCRAM-SHA-256 full handshake test (register → start → compute proof → finish)
- SCRAM-SHA-256 invalid proof rejection test

Build: 0 warnings, all tests pass

PLAN.md

@@ -104,7 +104,7 @@
 - Моделира sync между document/vector/graph/FTS индекси
 - Резултат: 10-ти TLA+ спек, пълно покритие на core модулите
 
-### Опция C: Auth hardening + SCRAM (3-4 часа)
+### Опция C: Auth hardening + SCRAM ✅
 - Истински SCRAM-SHA-256 със salt, iteration count, challenge-response
 - Резултат: production-grade auth