feat(auth): introduce backend auth module

Port RFC-001 authentication core from PR #1728:
- JWT token handling (create_access_token, decode_token, TokenPayload)
- Password hashing (bcrypt) with verify_password
- SQLite UserRepository with base interface
- Provider Factory pattern (LocalAuthProvider)
- CLI reset_admin tool
- Auth-specific errors (AuthErrorCode, TokenError, AuthErrorResponse)

Deps:
- bcrypt>=4.0.0
- pyjwt>=2.9.0
- email-validator>=2.0.0
- backend/uv.toml pins public PyPI index

Tests: 12 pure unit tests (test_auth_config.py, test_auth_errors.py).

Scope note: authz.py, test_auth.py, and test_auth_type_system.py are
deferred to commit 2 because they depend on middleware and deps wiring
that is not yet in place. Commit 1 stays "pure new files only" as the
spec mandates.

backend/app/gateway/auth/models.py

@@ -0,0 +1,41 @@
+"""User Pydantic models for authentication."""
+
+from datetime import UTC, datetime
+from typing import Literal
+from uuid import UUID, uuid4
+
+from pydantic import BaseModel, ConfigDict, EmailStr, Field
+
+
+def _utc_now() -> datetime:
+    """Return current UTC time (timezone-aware)."""
+    return datetime.now(UTC)
+
+
+class User(BaseModel):
+    """Internal user representation."""
+
+    model_config = ConfigDict(from_attributes=True)
+
+    id: UUID = Field(default_factory=uuid4, description="Primary key")
+    email: EmailStr = Field(..., description="Unique email address")
+    password_hash: str | None = Field(None, description="bcrypt hash, nullable for OAuth users")
+    system_role: Literal["admin", "user"] = Field(default="user")
+    created_at: datetime = Field(default_factory=_utc_now)
+
+    # OAuth linkage (optional)
+    oauth_provider: str | None = Field(None, description="e.g. 'github', 'google'")
+    oauth_id: str | None = Field(None, description="User ID from OAuth provider")
+
+    # Auth lifecycle
+    needs_setup: bool = Field(default=False, description="True for auto-created admin until setup completes")
+    token_version: int = Field(default=0, description="Incremented on password change to invalidate old JWTs")
+
+
+class UserResponse(BaseModel):
+    """Response model for user info endpoint."""
+
+    id: str
+    email: str
+    system_role: Literal["admin", "user"]
+    needs_setup: bool = False