feat(admin): audit-log viewer

Surface the admin_audit table (written by every mutating admin action but
previously unreadable) as a new /admin/audit tab. New repo::admin_audit::list
(LEFT JOIN users for the actor's username, filter by action/target_kind/actor/
since, at DESC via admin_audit_at_idx) behind GET /v1/admin/audit, mirroring the
crawler history endpoint's paged/clamped idiom.

Frontend: a self-contained AuditTable (target-kind + window + action filters,
expandable JSON payload, AbortController-cancelled fetches, loading/error/empty
states) and shared fmtAgo() in format.ts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/repo/admin_audit.rs

@@ -1,14 +1,98 @@
-//! Admin-action audit log writes.
+//! Admin-action audit log writes + the admin-facing read query.
 //!
 //! Insert is always called from inside the same transaction as the
 //! action it audits — the executor parameter is `PgExecutor` so the
-//! caller passes `&mut *tx` directly.
+//! caller passes `&mut *tx` directly. [`list`] backs the audit-log viewer.
 
-use sqlx::PgExecutor;
+use chrono::{DateTime, Utc};
+use serde::Serialize;
+use sqlx::{FromRow, PgExecutor, PgPool};
 use uuid::Uuid;
 
 use crate::error::AppResult;
 
+/// One audit row joined to the actor's current username (NULL when the
+/// actor account was deleted — `actor_user_id` is `ON DELETE SET NULL`).
+#[derive(Debug, Clone, Serialize, FromRow)]
+pub struct AuditEntryRow {
+    pub id: Uuid,
+    pub actor_user_id: Option<Uuid>,
+    pub actor_username: Option<String>,
+    pub action: String,
+    pub target_kind: String,
+    pub target_id: Option<Uuid>,
+    pub payload: serde_json::Value,
+    pub at: DateTime<Utc>,
+}
+
+/// Optional filters for [`list`]. `None`/absent widens the scope; an
+/// empty-string action/target_kind is treated as absent by the caller.
+#[derive(Debug, Default, Clone)]
+pub struct AuditFilter<'a> {
+    pub action: Option<&'a str>,
+    pub target_kind: Option<&'a str>,
+    pub actor_user_id: Option<Uuid>,
+    pub since: Option<DateTime<Utc>>,
+}
+
+/// Paginated, newest-first audit log. Returns the page slice plus the
+/// filtered total. Ordering by `at DESC` uses `admin_audit_at_idx`.
+pub async fn list(
+    pool: &PgPool,
+    filter: AuditFilter<'_>,
+    limit: i64,
+    offset: i64,
+) -> AppResult<(Vec<AuditEntryRow>, i64)> {
+    let items = sqlx::query_as::<_, AuditEntryRow>(
+        r#"
+        SELECT
+            a.id,
+            a.actor_user_id,
+            u.username        AS actor_username,
+            a.action,
+            a.target_kind,
+            a.target_id,
+            a.payload,
+            a.at
+          FROM admin_audit a
+          LEFT JOIN users u ON u.id = a.actor_user_id
+         WHERE ($1::text IS NULL OR a.action = $1)
+           AND ($2::text IS NULL OR a.target_kind = $2)
+           AND ($3::uuid IS NULL OR a.actor_user_id = $3)
+           AND ($4::timestamptz IS NULL OR a.at >= $4)
+         ORDER BY a.at DESC
+         LIMIT $5 OFFSET $6
+        "#,
+    )
+    .bind(filter.action)
+    .bind(filter.target_kind)
+    .bind(filter.actor_user_id)
+    .bind(filter.since)
+    .bind(limit)
+    .bind(offset)
+    .fetch_all(pool)
+    .await?;
+
+    let (total,): (i64,) = sqlx::query_as(
+        r#"
+        SELECT COUNT(*)
+          FROM admin_audit a
+         WHERE ($1::text IS NULL OR a.action = $1)
+           AND ($2::text IS NULL OR a.target_kind = $2)
+           AND ($3::uuid IS NULL OR a.actor_user_id = $3)
+           AND ($4::timestamptz IS NULL OR a.at >= $4)
+        "#,
+    )
+    .bind(filter.action)
+    .bind(filter.target_kind)
+    .bind(filter.actor_user_id)
+    .bind(filter.since)
+    .fetch_one(pool)
+    .await?;
+
+    Ok((items, total))
+}
+
 pub async fn insert<'e, E: PgExecutor<'e>>(
     executor: E,
     actor_user_id: Uuid,