feat: apikey alias/renewal + markdown/patch authorship

- APIKey.alias (unique, required). Creating with an existing alias
  renews that key: same key string kept, validity reset to 15d,
  reactivated, name/roles updated (response has renewed=true).
- get_actor(): X-API-Key -> key alias, Bearer -> 'admin'.
- markdown & patch create/update record author / created_at /
  updated_at / last_modified_by from the actor.
- Idempotent run_migrations() (information_schema-guarded ALTERs +
  backfill) so existing tables/data gain the new columns on startup;
  create_all still covers fresh DBs.

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

db/models/MarkdownPatch.py

@@ -17,6 +17,9 @@ class MarkdownPatch(Base):
     )
     title = Column(String(255), nullable=True)
     content = Column(Text, nullable=False)
+    # Actor strings: alias of the API key, or 'admin' for KC-logged-in UI.
+    author = Column(String(255), nullable=True)
+    last_modified_by = Column(String(255), nullable=True)
     order = Column(Integer, default=0)
     created_at = Column(DateTime, default=lambda: datetime.datetime.now(datetime.UTC))
     updated_at = Column(
@@ -31,6 +34,8 @@ class MarkdownPatch(Base):
             'markdown_id': self.markdown_id,
             'title': self.title,
             'content': self.content,
+            'author': self.author,
+            'last_modified_by': self.last_modified_by,
             'order': self.order,
             'created_at': self.created_at,
             'updated_at': self.updated_at,