feat(odin): ai_conversations table + conversation_id on messages

prisma/migrations/20260608140000_add_ai_conversations/migration.sql

@@ -0,0 +1,32 @@
+-- Multi-conversation Odin: conversations table + conversation_id on messages.
+
+CREATE TABLE `ai_conversations` (
+  `id` INTEGER NOT NULL AUTO_INCREMENT,
+  `user_id` INTEGER NOT NULL,
+  `title` VARCHAR(255) NOT NULL,
+  `created_at` TIMESTAMP(0) NOT NULL DEFAULT CURRENT_TIMESTAMP(0),
+  `updated_at` TIMESTAMP(0) NOT NULL DEFAULT CURRENT_TIMESTAMP(0),
+  PRIMARY KEY (`id`),
+  INDEX `idx_ai_conv_user` (`user_id`, `id`)
+) DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
+
+ALTER TABLE `ai_chat_messages`
+  ADD COLUMN `conversation_id` INTEGER NULL AFTER `user_id`;
+
+-- Backfill: one conversation per user who has messages ("keep as first conversation").
+INSERT INTO `ai_conversations` (`user_id`, `title`, `created_at`, `updated_at`)
+SELECT `user_id`, 'Konverzace', NOW(), NOW()
+FROM `ai_chat_messages`
+GROUP BY `user_id`;
+
+UPDATE `ai_chat_messages` m
+JOIN `ai_conversations` c ON c.`user_id` = m.`user_id`
+SET m.`conversation_id` = c.`id`;
+
+-- Enforce: drop old per-user index, NOT NULL, FK (cascade), per-conversation index.
+ALTER TABLE `ai_chat_messages`
+  DROP INDEX `idx_ai_chat_user`,
+  MODIFY `conversation_id` INTEGER NOT NULL,
+  ADD CONSTRAINT `fk_ai_chat_conv` FOREIGN KEY (`conversation_id`)
+    REFERENCES `ai_conversations`(`id`) ON DELETE CASCADE,
+  ADD INDEX `idx_ai_chat_conv` (`conversation_id`, `id`);