feat: unify Codex third-party providers into stable "custom" history bucket

Codex filters resume history by `model_provider`, so switching between
provider-specific ids like `rightcode` and `aihubmix` made past sessions
appear to vanish. Collapse all third-party providers into a single
stable bucket so cross-switch history stays visible.

- Normalize live `model_provider` to "custom" on every Codex write
  (reserved built-in ids like openai/ollama are preserved).
- Add device-level one-shot migration that rewrites historical JSONL
  session files and the `state_5.sqlite` threads table from legacy
  provider ids into the "custom" bucket. Backs up originals under
  `~/.cc-switch/backups/codex-history-provider-migration-v1/` and uses
  the SQLite Backup API for the state DB.
- Record completion in `settings.json` under `localMigrations` so the
  migration is strictly idempotent across launches.
- Update Codex provider preset templates to emit `model_provider = "custom"`
  out of the box.

src/lib/schemas/settings.ts

@@ -56,6 +56,21 @@ export const settingsSchema = z.object({
         .optional(),
     })
     .optional(),
+
+  // 本机自动迁移状态（后端维护，前端保存设置时应透传）
+  localMigrations: z
+    .object({
+      codexThirdPartyHistoryProviderBucketV1: z
+        .object({
+          completedAt: z.string(),
+          targetProviderId: z.string(),
+          sourceProviderIds: z.array(z.string()).optional(),
+          migratedJsonlFiles: z.number().optional(),
+          migratedStateRows: z.number().optional(),
+        })
+        .optional(),
+    })
+    .optional(),
 });
 
 export type SettingsFormData = z.infer<typeof settingsSchema>;