fix(database): exclude derived FTS search tables and generated columns from JSON export and import #1273

Summary

Schema 115 (#1204) added external-content FTS5 virtual tables (orders_search_fts, customers_search_fts) and their SQLite FTS5 shadow tables (*_fts_data, *_fts_idx, *_fts_docsize, *_fts_config). While getTables(db) in main/db.ts:2440 excludes virtual and FTS shadow tables, GET /api/db/export (main/routes/database.ts:93) queries raw sqlite_master tables and exports all 10 FTS virtual/shadow tables (plus customers.phone_digits virtual generated column). Re-importing that JSON via POST /api/db/import fails with SqliteError: table orders_search_fts_data may not be modified (or cannot INSERT into generated column "phone_digits").

Acceptance Criteria

• Reuse canonical getTables(db) (and stored getTableColumns(db, tableName)) in GET /api/db/export so derived FTS virtual/shadow tables and virtual generated columns are excluded from exports while preserving EXPORT_EXCLUDE_TABLES and secret redaction.
• Filter incoming tables in POST /api/db/import against getTables(db) and always intersect imported columns with getTableColumns(db, tableName) so existing schema 115 exports containing *_search_fts* shadow tables or customers.phone_digits import cleanly in both overwrite: true and overwrite: false modes, followed by rebuildSearchIndex(db) and executeSequenceRepair(db).
• Add full export -> overwrite and merge import round-trip tests at schema 115, schema 114 -> 115 import tests, contaminated schema 115 export tests, FTS searchability checks, and rollback sentinel checks.