fix(chain): persist first_seen

[LagginTimes]

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:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo +nightly fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

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

[evanlinjin]

ACK b27a019