Add question ownership and sharing

Questions now have a created_by field linking to the user who created them.
Users only see questions they own or that have been shared with them.
Includes share dialog, user search, bulk sharing, and export/import
respects ownership.

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

migrations/versions/d2113a61fa42_merge_heads.py

@@ -0,0 +1,24 @@
+"""Merge heads
+
+Revision ID: d2113a61fa42
+Revises: 2937635f309a, a1b2c3d4e5f6
+Create Date: 2026-04-03 09:32:19.765953
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = 'd2113a61fa42'
+down_revision = ('2937635f309a', 'a1b2c3d4e5f6')
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    pass
+
+
+def downgrade():
+    pass