refactor(persistence): consolidate monitoring index migrations

- Merge migrations 0039 through 0041 into a single 0039 migration
- Preserve existing indexes and support partial historical schemas
- Update migration tests for the consolidated revision

src/langbot/pkg/persistence/alembic/versions/0039_execution_lease_indexes.py

@@ -1,4 +1,4 @@
-"""Bound host lease maintenance to active executions within each Workspace."""
+"""Index execution leases, conversation paging, and historical inline image expiry."""
 
 from alembic import op
 import sqlalchemy as sa
@@ -8,26 +8,40 @@ down_revision = '0038_llm_call_details'
 branch_labels = None
 depends_on = None
 
+INDEXES = (
+    ('agent_run', 'ix_agent_run_workspace_status_lease', ['workspace_id', 'status', 'execution_lease_expires_at']),
+    (
+        'monitoring_messages',
+        'ix_monitoring_messages_workspace_status_lease',
+        ['workspace_uuid', 'status', 'execution_lease_expires_at'],
+    ),
+    (
+        'monitoring_messages',
+        'ix_monitoring_messages_conversation_page',
+        ['workspace_uuid', 'bot_id', 'session_id', 'timestamp', 'id'],
+    ),
+    ('monitoring_messages', 'ix_monitoring_messages_media_ttl', ['workspace_uuid', 'timestamp', 'id']),
+    ('event_log', 'ix_event_log_media_ttl', ['workspace_id', 'created_at', 'id']),
+    ('agent_run', 'ix_agent_run_media_ttl', ['workspace_id', 'created_at', 'id']),
+    ('agent_run_event', 'ix_agent_run_event_media_ttl', ['created_at', 'id', 'run_id']),
+    ('transcript', 'ix_transcript_media_ttl', ['workspace_id', 'created_at', 'id']),
+)
+
 
 def upgrade():
     inspector = sa.inspect(op.get_bind())
-    for table, workspace_column in (
-        ('agent_run', 'workspace_id'),
-        ('monitoring_messages', 'workspace_uuid'),
-    ):
+    for table, index, columns in INDEXES:
         if not inspector.has_table(table):
             continue
-        index = f'ix_{table}_workspace_status_lease'
         # Fresh installations create these indexes from ORM metadata before migrations.
         if index not in {item['name'] for item in inspector.get_indexes(table)}:
-            op.create_index(index, table, [workspace_column, 'status', 'execution_lease_expires_at'])
+            op.create_index(index, table, columns)
 
 
 def downgrade():
     inspector = sa.inspect(op.get_bind())
-    for table in ('agent_run', 'monitoring_messages'):
+    for table, index, _ in reversed(INDEXES):
         if not inspector.has_table(table):
             continue
-        index = f'ix_{table}_workspace_status_lease'
         if index in {item['name'] for item in inspector.get_indexes(table)}:
             op.drop_index(index, table_name=table)

src/langbot/pkg/persistence/alembic/versions/0040_conversation_page_index.py

@@ -1,25 +0,0 @@
-"""Page conversation identities without reading historical message payloads."""
-
-from alembic import op
-import sqlalchemy as sa
-
-revision = '0040_conversation_page_index'
-down_revision = '0039_execution_lease_indexes'
-branch_labels = None
-depends_on = None
-
-TABLE = 'monitoring_messages'
-INDEX = 'ix_monitoring_messages_conversation_page'
-
-
-def upgrade():
-    inspector = sa.inspect(op.get_bind())
-    # Fresh databases already receive this index from ORM metadata.
-    if inspector.has_table(TABLE) and INDEX not in {index['name'] for index in inspector.get_indexes(TABLE)}:
-        op.create_index(INDEX, TABLE, ['workspace_uuid', 'bot_id', 'session_id', 'timestamp', 'id'])
-
-
-def downgrade():
-    inspector = sa.inspect(op.get_bind())
-    if inspector.has_table(TABLE) and INDEX in {index['name'] for index in inspector.get_indexes(TABLE)}:
-        op.drop_index(INDEX, table_name=TABLE)

src/langbot/pkg/persistence/alembic/versions/0041_inline_media_ttl_indexes.py

@@ -1,37 +0,0 @@
-"""Index incremental expiry of historical inline images."""
-
-from alembic import op
-import sqlalchemy as sa
-
-revision = '0041_inline_media_ttl_indexes'
-down_revision = '0040_conversation_page_index'
-branch_labels = None
-depends_on = None
-
-INDEXES = (
-    ('monitoring_messages', ['workspace_uuid', 'timestamp', 'id']),
-    ('event_log', ['workspace_id', 'created_at', 'id']),
-    ('agent_run', ['workspace_id', 'created_at', 'id']),
-    ('agent_run_event', ['created_at', 'id', 'run_id']),
-    ('transcript', ['workspace_id', 'created_at', 'id']),
-)
-
-
-def upgrade():
-    inspector = sa.inspect(op.get_bind())
-    for table, columns in INDEXES:
-        if not inspector.has_table(table):
-            continue
-        name = f'ix_{table}_media_ttl'
-        if name not in {index['name'] for index in inspector.get_indexes(table)}:
-            op.create_index(name, table, columns)
-
-
-def downgrade():
-    inspector = sa.inspect(op.get_bind())
-    for table, _ in reversed(INDEXES):
-        if not inspector.has_table(table):
-            continue
-        name = f'ix_{table}_media_ttl'
-        if name in {index['name'] for index in inspector.get_indexes(table)}:
-            op.drop_index(name, table_name=table)