Verify installation, upgrades and concurrent intake across Drupal database drivers

Problem/Motivation

Alpha2 verification uses PostgreSQL 16. The README explicitly leaves MySQL and SQLite query plans and concurrency behavior unverified. The insert/savepoint/retry and conditional suppression-update paths need driver-specific evidence; this is a coverage gap, not a confirmed database defect.

Proposed resolution

Add isolated MySQL, MariaDB and SQLite fixtures using versions supported by the selected Drupal core. Retain PostgreSQL coverage and use a risk-based matrix rather than an unnecessary full Cartesian product.

Acceptance criteria

• Verify fresh install and alpha1-to-current batched upgrades on each driver, including more than 250 legacy rows and resumed updates.
• Race identical and distinct events, verify one logical intake per identity, newest suppression evidence and no partial event/state commits.
• Verify case normalization, unique-key behavior, unrelated database failures, privacy erasure and retention boundaries.
• Inspect suppression lookup and cleanup query plans on representative data; document measured scale and transaction behavior.
• Run automated regression coverage in CI and publish exact database versions and remaining limits.

Scope and dependencies

Stable-release acceptance task under #3621224: Complete the 1.0.0 stable release verification gates. Builds on #3621120: Make webhook retry deduplication atomic; the completed original issue remains fixed.