Index: docs/P3-UseCaseModel/wiki/UseCase0001.md
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+= Use-case 0001 — Register new account =
+
+'''Initiating actor:''' Visitor
+
+'''Other actors:''' —
+
+A new person creates an account on !EduBerza so they can later log in as a Trader. The system validates input, refuses duplicates, and stores a hashed password.
+
+== Scenario ==
+
+ 1. Visitor chooses "Register" from the anonymous menu.
+ 2. System prompts for username, email, full name and password.
+ 3. Visitor enters values.
+ 4. System validates:
+   * username, email, password are non-empty.
+   * email contains `@`.
+   * password is at least 6 characters.
+ 5. System checks whether the chosen username or email already exists:
+
+{{{
+SELECT EXISTS (
+    SELECT 1 FROM project.users
+     WHERE username = $1 OR email = $2
+);
+}}}
+
+ 6. If the row exists, system informs the user and the scenario ends. Otherwise it creates the account:
+
+{{{
+INSERT INTO project.users (username, email, full_name, password_hash, available_balance)
+VALUES ($1, $2, $3, encode(digest($4, 'sha256'), 'hex'), 0);
+}}}
+
+ 7. System confirms success and returns to the anonymous menu; Visitor can then proceed to UC0002.
+
+=== Alternate flow 3a — invalid email ===
+
+If step 4 fails email validation, system shows "Invalid email." and scenario returns to step 2.
+
+=== Alternate flow 5a — duplicate ===
+
+If step 5 returns `true`, system shows "Username or email already taken." and scenario ends.