Send a birthday or anniversary discount automatically

A store owner describes their process like this: "On a customer's birthday, I want
them emailed a 15% off coupon good for that week, and on the anniversary of their
first order, a separate 'thank you for a year with us' email with a smaller 10%
coupon."

AVAILABLE TOOLS: [Customer database with birthday field (trigger), Uncanny Automator,
WooCommerce coupons, email/SMTP]

Turn this into an implementation-ready automation recipe, thinking through what the
plain description leaves ambiguous:
1. Trigger: a daily scheduled check against birthday and first-order-anniversary
   fields, and the exact timezone to run it in, since running in server UTC time
   could send a customer's "birthday" email a day early or late depending on where
   they live.
2. Conditions: decide what happens for the sizable share of customers with no
   birthday on file, and confirm the recipe simply skips them rather than sending a
   broken email with an empty name-day field.
3. Actions in order: generate a unique, single-use coupon code per customer rather
   than reusing one shared code, since a shared birthday code posted online by one
   customer would get used by people it wasn't meant for, then send the email with an
   explicit expiry date in the subject line.
4. Failure handling: if coupon generation fails for one customer in a daily batch of
   thousands, should the whole batch halt, or should that one customer be logged and
   retried the next day without blocking everyone else's email.
5. One edge case specific to this exact workflow: a customer whose first order was
   also refunded in full shortly after. Decide whether they still count as having an
   "anniversary" worth celebrating, or whether the anniversary trigger should exclude
   fully refunded first orders.

Replace the workflow description and available tools with your own.