fix: waveform can generate NaN in json database (#481)

* refactor: fixes transcript duration type, NaN in waveform, and prepare for postgres migration

* fix: ensure we don't have NaN in waveform

* fix: missing assertionerror

Co-authored-by: pr-agent-monadical[bot] <198624643+pr-agent-monadical[bot]@users.noreply.github.com>

* fix: potential empty array

---------

Co-authored-by: pr-agent-monadical[bot] <198624643+pr-agent-monadical[bot]@users.noreply.github.com>

server/migrations/versions/a9c9c229ee36_transcript_composite_index.py

@@ -0,0 +1,39 @@
+"""transcript composite index
+
+Revision ID: a9c9c229ee36
+Revises: 88d292678ba2
+Create Date: 2025-07-15 20:09:40.253018
+
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+# revision identifiers, used by Alembic.
+revision: str = "a9c9c229ee36"
+down_revision: Union[str, None] = "88d292678ba2"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    with op.batch_alter_table("transcript", schema=None) as batch_op:
+        batch_op.create_index(
+            "idx_transcript_user_id_recording_id",
+            ["user_id", "recording_id"],
+            unique=False,
+        )
+
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    with op.batch_alter_table("transcript", schema=None) as batch_op:
+        batch_op.drop_index("idx_transcript_user_id_recording_id")
+
+    # ### end Alembic commands ###