feat: add email open analytics

[katsar0v]

Summary

• add a schema migration for per-recipient tracking tokens, first-open timestamps, and pixel-load counts
• inject a unique 1×1 tracking pixel into queued campaigns, scheduled one-time emails, and immediate one-time emails
• expose a cache-disabled public pixel endpoint that records opens without retaining IP addresses or user-agent strings
• show sent totals, unique opens, open rates, load counts, and per-recipient sent/open timestamps in the existing Queue screens
• add Bulgarian and German translations plus user-facing documentation of image blocking, privacy proxy, and prefetch limitations

Why

The queue already records delivery status and sent_at, but administrators have no built-in signal for whether an email client loaded a campaign. This provides first-party open analytics without requiring an external email platform.

An “open” means the recipient's email client or privacy proxy requested the remote image; it is intentionally presented as an approximate signal rather than proof that the recipient read the message.

Validation

• php vendor/bin/phpunit --configuration tests/phpunit.xml — 212 tests, 448 assertions
• php vendor/bin/phpcs --standard=phpcs.xml — passes all 48 scanned files
• wp i18n make-mo languages/ — Bulgarian and German MO output is reproducible
• Sass compilation of admin/scss/main.scss — passes

Closes #111