Add missing database indexes for performance

- Add index on quotes.chat_id: all queries filter by this column
- Add index on reminders.time: used in ORDER BY clauses
- Add composite indexes on reminders(chat_id, time) and
  reminders(user_id, time) for optimal query performance

Author: claude

model/sql/migrations/0016-add-missing-indexes.sql

@@ -0,0 +1,20 @@
+-- +migrate Up
+
+-- Add index on quotes.chat_id for better filtering performance
+-- All quote queries filter by chat_id
+ALTER TABLE `quotes`
+    ADD INDEX `chat_id` (`chat_id`);
+
+-- Add index on reminders.time for ORDER BY operations
+ALTER TABLE `reminders`
+    ADD INDEX `time` (`time`);
+
+-- Add composite index on reminders(chat_id, time)
+-- Optimizes: SELECT ... WHERE chat_id = ? ORDER BY time
+ALTER TABLE `reminders`
+    ADD INDEX `chat_id_time` (`chat_id`, `time`);
+
+-- Add composite index on reminders(user_id, time)
+-- Optimizes: SELECT ... WHERE chat_id IS NULL AND user_id = ? ORDER BY time
+ALTER TABLE `reminders`
+    ADD INDEX `user_id_time` (`user_id`, `time`);