Consolidate how Studio locates, detects, dumps and prepares the site database #5196

Studio's SQLite handling is spread across about 30 non-test files as repeated literals and calls. Consolidating it into a few helpers simplifies today's code and leaves a few clear places to branch if Studio ever supports another database engine.

Current state (trunk)

• The .ht.sqlite path is spelled out 18 times across 9 files: the importer, the Playground validator, pull-reprint, phpMyAdmin, blueprints, sqlite-collations, sqlite-journal-mode, config set and packages/common/lib/sqlite-integration.ts.
• Each of these infers the site's database in its own way:
  • config set checks for the database file;
  • the Playground validator looks for a database file and no SQL dumps;
  • Preserve external MySQL database names #4690 decides DB_NAME ownership from the SQLite integration state.
• wp sqlite export is run from three places in export-database.ts, and wp sqlite import is run separately from importer.ts.
• keepSqliteIntegrationUpdated is called from 8 places (create, start, export, import, push, blueprints, the importer), each with its own progress logging.
• The AI agent's WP-CLI tool description hardcodes "Studio sites use SQLite" (apps/cli/ai/tools/wp-cli.ts).

Plan

1. Database path: one getSiteDatabasePath( sitePath ) used by every caller. Stacked on Keep the SQLite database at a fixed path with DB_PATH #5188, which sets the fixed path through DB_PATH.
2. Database engine: one getSiteDatabaseEngine( sitePath ), derived from what's on disk; no new stored setting.
3. Dump and load: a single module with dumpDatabase( site ) and loadDatabase( site, files ) wraps the wp sqlite export/import calls. The collation normalisation stays inside it.
4. Preparation: one ensureSiteDatabaseReady( site ) replaces the scattered keepSqliteIntegrationUpdated calls and their logging.
5. AI tool text: the WP-CLI tool's database description comes from the engine helper.

Behaviour is unchanged throughout: same archives, same push format, same database location.