fix: close QA P1 findings on auto-schema-upgrade (#4)

Closes three P1 findings from the QA baseline review of a8a0ce2:
- uninstall.php deletes psa_db_version
- PSA_Upgrade class with unit tests (is_needed is pure, maybe_run is the driver)
- ARCHITECTURE.md documents the schema-versioning mechanism

Also extracts the logic into includes/class-psa-upgrade.php so it is cleanly testable without bootstrapping the whole plugin file.

Agent-Session: cowork-qa-p1-fixes-20260414

uninstall.php

@@ -42,6 +42,7 @@ if ( ! is_wp_error( $terms ) && ! empty( $terms ) ) {
 // 3. Delete plugin options.
 delete_option( 'psa_settings' );
 delete_option( 'psa_search_cache_gen' );
+delete_option( 'psa_db_version' );
 
 // 4. Delete all plugin transients (validation cache, rate limits).
 $wpdb->query(