Add inserted_at to lightning_columns

Summary

lightning_clients doesn't record when a client was provisioned, so we can't tell when a client was added, and a future client list command would have nothing to sort or display by.

The runner records each applied filename in _migrations and only runs files it hasn't recorded yet (platform/src/db/migrate.ts:40-45); that table is the source of truth, not the IF NOT EXISTS guards in the DDL (platform/src/db/migrate.ts:5-7).

Proposed change

• Add platform/migrations/0002_lightning_clients_inserted_at.sql with:

  ALTER TABLE lightning_clients
    ADD COLUMN IF NOT EXISTS inserted_at TIMESTAMPTZ NOT NULL DEFAULT now();
  

  Existing rows backfill to the migration time, which is fine for a provisioning timestamp.
• No change needed in store.ts: insertClient names its columns explicitly (name, auth_token_hash, anthropic_api_key), and DEFAULT now() fills in inserted_at on insert (platform/src/auth/client/store.ts:34-37). Verified.
• Extend the column assertions in db.test.ts to expect inserted_at (platform/test/db.test.ts:36-39).

Acceptance criteria

• platform/migrations/0002_lightning_clients_inserted_at.sql adds inserted_at TIMESTAMPTZ NOT NULL DEFAULT now() via ALTER TABLE ... ADD COLUMN IF NOT EXISTS
• 0001_lightning_clients.sql is left untouched
• bun run migrate applies 0002 against a DB that already has 0001 recorded, and is a no-op on a second run (platform/src/db/migrate.ts:42-47)
• Existing rows have inserted_at backfilled to the migration time
• db.test.ts asserts inserted_at is present in lightning_clients (platform/test/db.test.ts:36-39)
• No change required to insertClient in store.ts (confirmed the default populates it)

Start with platform/src/db/migrate.ts:40-47 and platform/test/db.test.ts:36-39, then inspect the existing migration format in platform/migrations/0001_lightning_clients.sql. Run bun run migrate against a database with 0001 recorded and execute the database tests. Done means 0002 applies once, existing rows receive inserted_at, and the column assertion passes without changes to store.ts.