Miscellaneous fixes (#24179)

* rename migration

* use ASC composite index for event camera and start_time

the planner still picks for newest-first single-camera queries but doesn't slow down full per-camera scans on a cold cache

* pause playback while the timeline range handles are up

* reduce multi camera seeded export range to 30m

allows both handlebars to fit within most desktop windows

migrations/039_add_perf_indexes.py

@@ -0,0 +1,21 @@
+"""Peewee migrations -- 039_add_perf_indexes.py.
+
+Adds a composite (camera, start_time) index to speed up single-camera queries
+issued by the web UI.
+
+"""
+
+import peewee as pw
+
+SQL = pw.SQL
+
+
+def migrate(migrator, database, fake=False, **kwargs):
+    migrator.sql(
+        'CREATE INDEX IF NOT EXISTS "event_camera_start_time" '
+        'ON "event" ("camera", "start_time")'
+    )
+
+
+def rollback(migrator, database, fake=False, **kwargs):
+    migrator.sql('DROP INDEX IF EXISTS "event_camera_start_time"')