schema: relax email UNIQUE to partial unique index for non-guest users

Allows multiple guest accounts to share an email while enforcing
uniqueness for registered users. Enables the disposable guest account pattern.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

init-scripts/init-script.sql

@@ -88,7 +88,8 @@ CREATE TABLE users (
     n_first_name VARCHAR(50) NOT NULL,          -- vCard N.given
     n_last_name VARCHAR(50) NOT NULL,           -- vCard N.family
     fn VARCHAR(120) GENERATED ALWAYS AS (n_first_name || ' ' || n_last_name) STORED, -- vCard FN
-    email VARCHAR(255) UNIQUE,                 -- vCard EMAIL (nullable for social-only/guest)
+    email VARCHAR(255),                 -- vCard EMAIL (nullable for social-only/guest)
+                                       -- Uniqueness enforced only for non-guest users via partial index
     phone VARCHAR(20) NOT NULL,                         -- vCard TEL
     date_of_birth DATE NOT NULL,                        -- vCard BDAY
     profile_pic_url TEXT,                      -- vCard PHOTO
@@ -131,6 +132,10 @@ CREATE TABLE user_social_logins (
 CREATE INDEX idx_users_email_lower ON users (LOWER(email));
 CREATE INDEX idx_users_account_role ON users (account_role);
 
+-- Enforce unique email only for registered (non-guest) users
+-- Guest accounts can share emails; registered accounts cannot
+CREATE UNIQUE INDEX idx_users_email_registered ON users (email) WHERE account_role != 'guest';
+
 -- =======================================
 -- VERIFICATION CODES TABLE
 -- =======================================