refactor(db): flatten migrations into a 2.0.0 baseline

[MrAlders0n]

Replaces the 1.x migration chain (001–045) with a single 001_baseline.sql for 2.0.0.

• Baseline is a schema-only dump of a fresh database migrated through 045, with materialized views created WITH DATA (so REFRESH ... CONCURRENTLY works) and view bodies taken from their original migrations so sqlc generate output is unchanged.
• Startup now refuses a pre-2.0.0 database (1.x ledger, or tables with no ledger) instead of applying the baseline on top. 2.0.0 needs a fresh database.
• Integration tests build their schema from the baseline; tests for one-time 1.x upgrades are removed. New tests cover baseline apply, idempotent rerun and the refusal.