Security audit: fix IDOR, add rate limiting, HMAC auth, token hashing, XSS sanitization

Critical: Add ownership checks to all order routes (IDOR fix).
High: Redis rate limiting on auth endpoints, HMAC-signed internal
service calls replacing header-presence-only checks, nh3 HTML
sanitization on ghost_sync and product import, internal auth on
market API endpoints, SHA-256 hashed OAuth grant/code tokens.
Medium: SECRET_KEY production guard, AP signature enforcement,
is_admin param removal, cart_sid validation, SSRF protection on
remote actor fetch.

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

shared/models/oauth_code.py

@@ -1,4 +1,5 @@
 from __future__ import annotations
+import hashlib
 from datetime import datetime
 from sqlalchemy import String, Integer, DateTime, ForeignKey, func, Index
 from sqlalchemy.orm import Mapped, mapped_column, relationship
@@ -6,21 +7,28 @@ from shared.db.base import Base
 
 
 class OAuthCode(Base):
+    """Short-lived authorization code issued during OAuth flow.
+
+    The ``code`` column is retained during migration but new codes store
+    only ``code_hash``.  Lookups should use ``code_hash``.
+    """
     __tablename__ = "oauth_codes"
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
-    code: Mapped[str] = mapped_column(String(128), unique=True, index=True, nullable=False)
+    code: Mapped[str | None] = mapped_column(String(128), nullable=True)
+    code_hash: Mapped[str | None] = mapped_column(String(64), unique=True, nullable=True, index=True)
     user_id: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True)
     client_id: Mapped[str] = mapped_column(String(64), nullable=False)
     redirect_uri: Mapped[str] = mapped_column(String(512), nullable=False)
     expires_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
     used_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
     grant_token: Mapped[str | None] = mapped_column(String(128), nullable=True)
+    grant_token_hash: Mapped[str | None] = mapped_column(String(64), nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())
 
     user = relationship("User", backref="oauth_codes")
 
     __table_args__ = (
-        Index("ix_oauth_code_code", "code", unique=True),
+        Index("ix_oauth_code_code_hash", "code_hash", unique=True),
         Index("ix_oauth_code_user", "user_id"),
     )