Merge bitcoindevkit/bdk#1966: fix(chain): persist `first_seen`

b27a019b595545ffe8fed6b77aeccb68f7b23938 fix(chain): persist `first_seen` (Wei Chen)

Pull request description:

  Fixes #1965.

  ### Description

  Adds missing persistence for `first_seen`, which was not included in #1950.

  ### Changelog notice

  - Adds `first_seen` column to the `bdk_txs` table via schema v3 migration.
  - Updates `from_sqlite()` and `persist_to_sqlite()` to handle `first_seen`.
  - Updates the v0-to-v3 migration test to verify compatibility with older schemas.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo +nightly fmt` and `cargo clippy` before committing

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've added docs for the new feature

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [x] I've added tests to reproduce the issue which are now passing
  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  evanlinjin:
    ACK b27a019b595545ffe8fed6b77aeccb68f7b23938

Tree-SHA512: a8c4cd930e20f7bdf1a02fc3155b5df9f1627676fe10a2d77ea856e71e45f783bba1bb8cf4eceb8dba71c345e7985a9e091002966cec147871e6672c0e2ac5c4