* receipt system

i couldn't think of a better name so i'll call them receipt. basically it's the "confirmation link sent through email" thing. the core thing about receipts is that it's a one-time thing that triggers certain action and the user shouldn't have any method of creating one out of thin air.

two main kinds of actions that would be triggered by a returned receipt would be:

+ registration confirmation
+ email confirmation
+ password reset

(this would obviously better be implemented w/ kv-stores where you can set a timer in entries like Redis; using sqlite is just for the ease of business logic development).

#+begin_src sql
  CREATE TABLE IF NOT EXISTS receipt (
    id TEXT UNIQUE,
    command TEXT,
    issue_time INT,
    timeout_minute INT,
  )
#+end_src

for receipt command's internal format see `pkg/gitus/receipt/rsif.go`.