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-tauri/src/database/mod.rs

@@ -32,7 +32,7 @@ mod schema;
 mod tests;
 
 // DAO 类型导出供外部使用
-pub(crate) use dao::providers_seed::CLAUDE_DESKTOP_OFFICIAL_PROVIDER_ID;
+pub(crate) use dao::providers_seed::{is_official_seed_id, CLAUDE_DESKTOP_OFFICIAL_PROVIDER_ID};
 pub(crate) use dao::proxy::{
     validate_cost_multiplier, validate_pricing_source, PRICING_SOURCE_REQUEST,
     PRICING_SOURCE_RESPONSE,