fix: add missing primary key constraints to legacy Peewee tables

Tables created by the Peewee migration system were created with only
UNIQUE constraints on their `id` columns, not PRIMARY KEY constraints.
Alembic's init migration skips these tables since they already exist,
so the PKs were never added. This migration retroactively promotes the
unique constraint to a primary key for: chat, chatidtag, file, function,
memory, model, tool.

The upgrade() is idempotent — it inspects the existing PK constraint
before acting. The redundant unique constraint on `id` is dropped first.

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

backend/open_webui/migrations/versions/c3d4e5f6a7b8_add_missing_primary_keys.py

@@ -0,0 +1,51 @@
+"""add missing primary keys to legacy peewee tables
+
+Revision ID: c3d4e5f6a7b8
+Revises: b2c3d4e5f6a7
+Create Date: 2026-03-13 00:00:00.000000
+
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+from sqlalchemy.engine.reflection import Inspector
+
+# revision identifiers, used by Alembic.
+revision: str = "c3d4e5f6a7b8"
+down_revision: Union[str, None] = "b2c3d4e5f6a7"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+TABLES = [
+    ("chat", "id"),
+    ("chatidtag", "id"),
+    ("file", "id"),
+    ("function", "id"),
+    ("memory", "id"),
+    ("model", "id"),
+    ("tool", "id"),
+]
+
+
+def upgrade() -> None:
+    conn = op.get_bind()
+    inspector = Inspector.from_engine(conn)
+
+    for table, column in TABLES:
+        pk = inspector.get_pk_constraint(table)
+        if column in (pk.get("constrained_columns") or []):
+            continue
+
+        unique_constraints = inspector.get_unique_constraints(table)
+        with op.batch_alter_table(table, schema=None) as batch_op:
+            for uc in unique_constraints:
+                if uc.get("column_names") == [column]:
+                    batch_op.drop_constraint(uc["name"], type_="unique")
+            batch_op.create_primary_key(f"pk_{column}", [column])
+
+
+def downgrade() -> None:
+    for table, column in TABLES:
+        with op.batch_alter_table(table, schema=None) as batch_op:
+            batch_op.drop_constraint(f"pk_{column}", type_="primary")