EastmarkHK Mailer — Hosted open tracking (mailer.emhk.shop)
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1. Upload this folder to:
   https://mailer.emhk.shop/EastmarkHK_Mailer/

2. Copy config.sample.php → config.php
   - Set public_base_url to https://mailer.emhk.shop/EastmarkHK_Mailer
   - Set provision_key to a long random string (same value embedded in the Mac app)

3. Ensure PHP can write to data/ (chmod 755 or 775).

4. In the Mac app → Settings → Open tracking:
   - Choose "EastmarkHK hosted"
   - Enable tracking — the app registers automatically (no URL/secret to copy)

Storage layout (automatic per customer)
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  data/tenants/{tenant_id}.json   — one file per Mac install
  data/by_install/{install_id}.txt  — maps install → tenant
  data/opens/{tenant_id}/opens.jsonl — open events per customer

Endpoints
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  POST provision.php              — auto-register (app only, X-Provision-Key)
  GET  track.php?t=…&id=…         — 1×1 pixel in emails
  GET  opens.php                  — sync for Mac app (X-Eastmark-Api-Secret)

Legacy single-tenant (optional)
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  Set api_secret in config.php for old self-hosted installs without tenant id.

Privacy: IP + user-agent stored per open, isolated per tenant.
