fix: cap /api/discovery/ping at 4096 IPs and fix test suite

- Add MAX_PING_IPS=4096 constant and validate list size in PingRequest
  before spawning futures, returning 422 on overflow
- Add test_ping_too_many_ips_rejected to cover the new cap
- Pin httpx<0.28 in requirements-test.txt (0.28 broke TestClient API)
- Fix reset_db fixture to set a known admin password regardless of
  INITIAL_ADMIN_PASSWORD env var (was causing 401 on all auth tests)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/test_validation.py

@@ -9,6 +9,7 @@ Couvre :
 """
 import pytest
 from fastapi.testclient import TestClient
+from passlib.context import CryptContext
 from sqlalchemy import text
 
 import sys, os
@@ -17,6 +18,9 @@ sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 from database import engine, Base
 from main import app, _migrate_users_must_change_password, _migrate_users_token_version, _migrate_users
 
+_TEST_PASSWORD = "admin-test"
+_pwd_ctx = CryptContext(schemes=["bcrypt"], deprecated="auto")
+
 
 # ---------------------------------------------------------------------------
 # Fixtures
@@ -29,6 +33,11 @@ def reset_db():
     _migrate_users_must_change_password()
     _migrate_users_token_version()
     _migrate_users()
+    # Force a known password regardless of INITIAL_ADMIN_PASSWORD env var
+    with engine.connect() as conn:
+        hashed = _pwd_ctx.hash(_TEST_PASSWORD)
+        conn.execute(text(f"UPDATE users SET hashed_password=:h WHERE username='admin'"), {"h": hashed})
+        conn.commit()
     yield
     Base.metadata.drop_all(bind=engine)
 
@@ -55,7 +64,7 @@ def _get_token(client):
     with engine.connect() as conn:
         conn.execute(text("UPDATE users SET must_change_password=0 WHERE username='admin'"))
         conn.commit()
-    r = client.post("/api/auth/login", data={"username": "admin", "password": "admin"})
+    r = client.post("/api/auth/login", data={"username": "admin", "password": _TEST_PASSWORD})
     return r.json()["access_token"]
 
 
@@ -189,6 +198,12 @@ class TestDiscoveryValidation:
         r = client.post("/api/discovery/ping", json={"ips": []}, headers=_auth(token))
         assert r.status_code == 200
 
+    def test_ping_too_many_ips_rejected(self, client):
+        token = _get_token(client)
+        ips = [f"10.0.{i // 256}.{i % 256}" for i in range(4097)]
+        r = client.post("/api/discovery/ping", json={"ips": ips}, headers=_auth(token))
+        assert r.status_code == 422
+
     def test_scan_oversized_cidr_rejected(self, client):
         token = _get_token(client)
         r = client.post("/api/discovery/scan", json={