Remove backwards compat code: no installed base pre-1.0

- Simplify bootstrapMigrationsTable to just check table existence and
  apply 000.sql if missing — no legacy DB detection
- Remove ensureBootstrapVersionRecorded (legacy backfill path)
- Remove applyBootstrapMigration (separate helper, now inlined)
- Remove TestBootstrapMigrationsTable_ExistingTableBackwardsCompat
- Simplify 000.sql header comment

internal/database/schema/000.sql

@@ -1,7 +1,5 @@
 -- Migration 000: Schema migrations tracking table
--- This must be the first migration applied. The bootstrap logic in
--- database.go applies it directly (bypassing the normal migration
--- loop) when the schema_migrations table does not yet exist.
+-- Applied as a bootstrap step before the normal migration loop.
 
 CREATE TABLE IF NOT EXISTS schema_migrations (
     version TEXT PRIMARY KEY,