Admin can now write posts

Posts can be displayed on the website. Subscribers are automatically
notified by email. This gives the opportunity to track explicitly how
many people followed the link provided in the emails sent without being
intrusive (no invisible image).

migrations/20250918120924_create_posts_table.sql

@@ -0,0 +1,7 @@
+CREATE TABLE posts (
+    post_id UUID PRIMARY KEY,
+    author_id UUID NOT NULL REFERENCES users (user_id),
+    title TEXT NOT NULL,
+    content TEXT NOT NULL,
+    published_at TIMESTAMPTZ NOT NULL
+);