Basic minimal prototype

Use a postgres database as storage and start with sone basic
functionality.

migrations/2023-01-18-102900_create_reminder/down.sql

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+DROP TABLE reminders;

migrations/2023-01-18-102900_create_reminder/up.sql

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+CREATE TABLE reminders (
+       id SERIAL NOT NULL PRIMARY KEY,
+       created TIMESTAMPTZ NOT NULL DEFAULT now(),
+       planned TIMESTAMPTZ NOT NULL,
+       executed TIMESTAMPTZ,
+       title TEXT NOT NULL,
+       message TEXT NOT NULL,
+       receiver TEXT NOT NULL
+);