Phase 3: Fix medium-priority issues (M1-M4, M6)

M1: Password complexity validation (min 8 chars, uppercase, lowercase, digit)
M2: Remove is_system_admin from login response (prevent role leaking)
M3: Permission cache invalidates on DB error instead of using stale data
M4: .env.docker.example already fixed in B9 (SECRET_KEY, FRONTEND_URL, SMTP)
M6: Frontend test setup auto-wraps with QueryClientProvider (fixes ~29 test failures)

Remaining: M5 (frontend component integration — WelcomeDialog, SavedFilterBar, etc.)

app/schemas/user.py

@@ -2,7 +2,24 @@
 
 from __future__ import annotations
 
-from pydantic import BaseModel, EmailStr, Field
+import re
+from pydantic import BaseModel, EmailStr, Field, field_validator
+
+
+def _validate_password_complexity(password: str) -> str:
+    """Validate password meets complexity requirements.
+
+    Requires: min 8 chars, at least 1 uppercase, 1 lowercase, 1 digit.
+    """
+    if len(password) < 8:
+        raise ValueError("Password must be at least 8 characters")
+    if not re.search(r"[A-Z]", password):
+        raise ValueError("Password must contain at least one uppercase letter")
+    if not re.search(r"[a-z]", password):
+        raise ValueError("Password must contain at least one lowercase letter")
+    if not re.search(r"\d", password):
+        raise ValueError("Password must contain at least one digit")
+    return password
 
 
 class UserCreate(BaseModel):
@@ -13,6 +30,11 @@ class UserCreate(BaseModel):
     role_id: str | None = Field(default=None, description="UUID of a custom Role", examples=["550e8400-e29b-41d4-a716-446655440000"])
     is_active: bool = True
 
+    @field_validator("password")
+    @classmethod
+    def validate_password(cls, v: str) -> str:
+        return _validate_password_complexity(v)
+
 
 class UserUpdate(BaseModel):
     name: str | None = Field(None, min_length=1, max_length=200)