feat(db): schema bootstrap with append-only audit_log table

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/db/migrations/0001_audit_log.sql

@@ -0,0 +1,28 @@
+-- Append-only audit log. One database == one organization, so there is no org_id.
+CREATE TABLE audit_log (
+    seq         BIGINT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
+    at          TIMESTAMPTZ NOT NULL DEFAULT now(),
+    actor_kind  TEXT  NOT NULL CHECK (actor_kind IN ('user', 'system')),
+    actor_id    UUID,
+    action      TEXT  NOT NULL CHECK (action IN ('created', 'updated', 'deleted')),
+    entity_type TEXT  NOT NULL,
+    entity_id   UUID  NOT NULL,
+    changes     JSONB NOT NULL DEFAULT '[]'::jsonb,
+    CONSTRAINT actor_id_matches_kind CHECK (
+        (actor_kind = 'user'   AND actor_id IS NOT NULL) OR
+        (actor_kind = 'system' AND actor_id IS NULL)
+    )
+);
+
+CREATE INDEX audit_log_entity_idx ON audit_log (entity_type, entity_id, seq);
+
+-- Enforce append-only at the database level: reject any UPDATE or DELETE.
+CREATE OR REPLACE FUNCTION audit_log_reject_mutation() RETURNS trigger AS $$
+BEGIN
+    RAISE EXCEPTION 'audit_log is append-only; % is not permitted', TG_OP;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER audit_log_immutable
+    BEFORE UPDATE OR DELETE ON audit_log
+    FOR EACH ROW EXECUTE FUNCTION audit_log_reject_mutation();

crates/db/src/lib.rs

@@ -37,4 +37,13 @@ impl Db {
 
         Ok(())
     }
+
+    /// Apply all pending schema migrations (embedded at compile time).
+    ///
+    /// Pre-1.0 the migration files are rewritten freely and dev databases are
+    /// recreated; this is the schema-bootstrap mechanism, not forward-migration
+    /// discipline.
+    pub async fn migrate(&self) -> Result<(), sqlx::migrate::MigrateError> {
+        sqlx::migrate!().run(&self.pool).await
+    }
 }

crates/db/tests/migrate.rs

@@ -0,0 +1,20 @@
+use db::Db;
+use sqlx::PgPool;
+
+#[sqlx::test]
+async fn migrate_is_idempotent_and_creates_audit_log(pool: PgPool) {
+    let db = Db::from_pool(pool);
+
+    // sqlx::test already applied migrations to this temp DB; re-running must be a
+    // no-op success (idempotent).
+    db.migrate()
+        .await
+        .expect("re-running migrate is idempotent");
+
+    let regclass: Option<String> =
+        sqlx::query_scalar("SELECT to_regclass('public.audit_log')::text")
+            .fetch_one(db.pool())
+            .await
+            .unwrap();
+    assert_eq!(regclass.as_deref(), Some("audit_log"));
+}