Bambuddy can now use an external PostgreSQL database via the
DATABASE_URL environment variable. SQLite remains the default.
Dialect-aware helpers handle upserts, PRAGMAs, FTS (FTS5 vs
tsvector+GIN), backup/restore, and health checks. All migration
blocks use savepoints to prevent Postgres transaction poisoning.
Backups are always portable SQLite format regardless of backend.
Cross-database restore imports SQLite backups into PostgreSQL
with automatic boolean/datetime conversion, NOT NULL default
filling, and FK constraint handling.
