feat: markdown patch cards (model + API)

Add MarkdownPatch model (markdown_patch table, auto-created by
create_all) and /api/patch blueprint: list patches for a markdown
(inherits the parent's private/protected visibility), create/update
(admin|creator), delete (admin).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

db/models/MarkdownPatch.py

@@ -0,0 +1,37 @@
+from sqlalchemy import Column, Text, Integer, String, DateTime, ForeignKey
+from db.models import Base
+import datetime
+
+
+class MarkdownPatch(Base):
+    """A markdown 'patch card' attached to a parent markdown document.
+
+    Its content is plain markdown text rendered below the parent's body.
+    Created/updated by admin|creator, deleted by admin; visibility is
+    inherited from the parent markdown (enforced in the API layer).
+    """
+    __tablename__ = 'markdown_patch'
+    id = Column(Integer, primary_key=True)
+    markdown_id = Column(
+        Integer, ForeignKey('markdown.id'), nullable=False, index=True
+    )
+    title = Column(String(255), nullable=True)
+    content = Column(Text, nullable=False)
+    order = Column(Integer, default=0)
+    created_at = Column(DateTime, default=lambda: datetime.datetime.now(datetime.UTC))
+    updated_at = Column(
+        DateTime,
+        default=lambda: datetime.datetime.now(datetime.UTC),
+        onupdate=lambda: datetime.datetime.now(datetime.UTC),
+    )
+
+    def to_dict(self):
+        return {
+            'id': self.id,
+            'markdown_id': self.markdown_id,
+            'title': self.title,
+            'content': self.content,
+            'order': self.order,
+            'created_at': self.created_at,
+            'updated_at': self.updated_at,
+        }