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/config.rs

@@ -59,6 +59,13 @@ pub struct Config {
     pub upload: UploadConfig,
     pub cors_allowed_origins: Vec<String>,
     pub crawler: CrawlerConfig,
+    /// `(username, password)` for the admin user provisioned at startup
+    /// when both `ADMIN_USERNAME` and `ADMIN_PASSWORD` are set. `None`
+    /// skips the bootstrap entirely. See `repo::user::bootstrap_admin`
+    /// for the create-vs-promote semantics — notably the password here
+    /// is used only when creating a new row, never to overwrite an
+    /// existing one.
+    pub admin_bootstrap: Option<(String, String)>,
 }
 
 /// All crawler-daemon knobs read from env. Mirrors the env vars the
@@ -158,10 +165,21 @@ impl Config {
                 })
                 .unwrap_or_default(),
             crawler: CrawlerConfig::from_env()?,
+            admin_bootstrap: admin_bootstrap_from_env(),
         })
     }
 }
 
+/// Returns `Some((username, password))` only when BOTH `ADMIN_USERNAME`
+/// and `ADMIN_PASSWORD` are set and non-empty. Half-set configuration is
+/// treated as "no bootstrap" rather than a hard error, so an operator
+/// can comment out one env var without crashing the server.
+fn admin_bootstrap_from_env() -> Option<(String, String)> {
+    let username = std::env::var("ADMIN_USERNAME").ok().filter(|s| !s.is_empty())?;
+    let password = std::env::var("ADMIN_PASSWORD").ok().filter(|s| !s.is_empty())?;
+    Some((username, password))
+}
+
 impl CrawlerConfig {
     pub fn from_env() -> anyhow::Result<Self> {
         // Parse CRAWLER_DAILY_AT (HH:MM, 24h). Invalid → fail fast.