feat(auth): admin role with cookie-only RequireAdmin extractor (0.37.0)

Adds an `is_admin` flag on users plus the substrate every later PR in the
admin feature builds on:

- migration 0018 adds the column with default false
- `repo::user::bootstrap_admin` creates or promotes the user named by
  `ADMIN_USERNAME` at startup, hashing `ADMIN_PASSWORD` only when the row
  is new — never overwriting an existing hash, so an operator can rotate
  the admin password via the UI without env-var conflict
- `CurrentSessionUser` extractor accepts only the session cookie;
  `RequireAdmin` composes over it and additionally requires
  `user.is_admin`. Bearer tokens are intentionally excluded so an
  admin's bot token never inherits admin authority (privilege-escalation
  surface that bites every "API keys reuse user perms" auth design)
- demotion is instant: `RequireAdmin` re-reads the user row each request

`/api/v1/auth/me` now exposes `is_admin`; no other response embeds
`User`, so no privacy fanout to audit.

backend/src/repo/user.rs

@@ -11,7 +11,7 @@ pub async fn create(pool: &PgPool, username: &str, password_hash: &str) -> AppRe
         r#"
         INSERT INTO users (username, password_hash)
         VALUES ($1, $2)
-        RETURNING id, username, password_hash, created_at
+        RETURNING id, username, password_hash, created_at, is_admin
         "#,
     )
     .bind(username)
@@ -35,7 +35,7 @@ pub async fn create(pool: &PgPool, username: &str, password_hash: &str) -> AppRe
 pub async fn find_by_username(pool: &PgPool, username: &str) -> AppResult<Option<User>> {
     let row = sqlx::query_as::<_, User>(
         r#"
-        SELECT id, username, password_hash, created_at
+        SELECT id, username, password_hash, created_at, is_admin
         FROM users
         WHERE lower(username) = lower($1)
         "#,
@@ -48,7 +48,7 @@ pub async fn find_by_username(pool: &PgPool, username: &str) -> AppResult<Option
 
 pub async fn find_by_id(pool: &PgPool, id: Uuid) -> AppResult<Option<User>> {
     let row = sqlx::query_as::<_, User>(
-        r#"SELECT id, username, password_hash, created_at FROM users WHERE id = $1"#,
+        r#"SELECT id, username, password_hash, created_at, is_admin FROM users WHERE id = $1"#,
     )
     .bind(id)
     .fetch_optional(pool)
@@ -56,3 +56,54 @@ pub async fn find_by_id(pool: &PgPool, id: Uuid) -> AppResult<Option<User>> {
     Ok(row)
 }
 
+pub async fn set_is_admin(pool: &PgPool, id: Uuid, value: bool) -> AppResult<()> {
+    sqlx::query("UPDATE users SET is_admin = $1 WHERE id = $2")
+        .bind(value)
+        .bind(id)
+        .execute(pool)
+        .await?;
+    Ok(())
+}
+
+/// Ensure the user `username` exists and is an admin. Called at startup
+/// from `app::build` when `ADMIN_USERNAME` / `ADMIN_PASSWORD` are set.
+///
+/// Semantics — see cross-cutting decision #2 in the feature plan:
+/// - If no row exists: create with the env-supplied password hashed via
+///   argon2id and `is_admin = true`.
+/// - If a row already exists: flip `is_admin` to true if needed; **never**
+///   touch the existing `password_hash`. Lets the operator rotate the
+///   admin password through the UI without env-var conflict.
+/// Wrapped in a transaction so a concurrent `register` for the same
+/// username can't slip an INSERT between the SELECT and UPDATE/INSERT.
+pub async fn bootstrap_admin(
+    pool: &PgPool,
+    username: &str,
+    password: &str,
+) -> AppResult<()> {
+    let mut tx = pool.begin().await?;
+    let existing: Option<(Uuid,)> = sqlx::query_as(
+        "SELECT id FROM users WHERE lower(username) = lower($1) FOR UPDATE",
+    )
+    .bind(username)
+    .fetch_optional(&mut *tx)
+    .await?;
+    match existing {
+        Some((id,)) => {
+            sqlx::query("UPDATE users SET is_admin = true WHERE id = $1 AND is_admin = false")
+                .bind(id)
+                .execute(&mut *tx)
+                .await?;
+        }
+        None => {
+            let hash = crate::auth::password::hash_password(password)?;
+            sqlx::query("INSERT INTO users (username, password_hash, is_admin) VALUES ($1, $2, true)")
+                .bind(username)
+                .bind(&hash)
+                .execute(&mut *tx)
+                .await?;
+        }
+    }
+    tx.commit().await?;
+    Ok(())
+}