perf: add index on memory.user_id

Memory queries filter by user_id (get_memories_by_user_id,
delete_memories_by_user_id) but the column had no index, forcing
a full table scan per request. Adds a btree index via Alembic
migration plus index=True on the model so fresh installs include it.

backend/open_webui/migrations/versions/a749ab2aa953_add_memory_user_id_index.py

@@ -0,0 +1,26 @@
+"""Add index on memory.user_id
+
+Revision ID: a749ab2aa953
+Revises: c1d2e3f4a5b6
+Create Date: 2026-04-17 00:00:00.000000
+
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+revision = 'a749ab2aa953'
+down_revision = 'c1d2e3f4a5b6'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # Memory.user_id is filtered on every per-user query
+    # (get_memories_by_user_id, delete_memories_by_user_id),
+    # so a btree index prevents full-table scans as the table grows.
+    op.create_index('memory_user_id_idx', 'memory', ['user_id'])
+
+
+def downgrade():
+    op.drop_index('memory_user_id_idx', table_name='memory')

backend/open_webui/models/memories.py

@@ -19,7 +19,7 @@ class Memory(Base):
     __tablename__ = 'memory'
 
     id = Column(String, primary_key=True, unique=True)
-    user_id = Column(String)
+    user_id = Column(String, index=True)
     content = Column(Text)
     updated_at = Column(BigInteger)
     created_at = Column(BigInteger)