feat: user-scoped webhook endpoint for multi-tenant isolation

Adds POST /api/webhooks/u/{user_id}/{path} — a user-scoped webhook
endpoint that filters the routine lookup by user_id, preventing
cross-user webhook triggering when paths collide.

The existing /api/webhooks/{path} endpoint remains unchanged for
backward compatibility in single-user deployments.

Changes:
- get_webhook_routine_by_path gains user_id: Option<&str> param
- Both postgres and libsql implementations add AND user_id = ? filter
  when user_id is provided
- New webhook_trigger_user_scoped_handler extracts (user_id, path)
  from URL and passes to shared fire_webhook_inner logic
- Route registered on public router (webhooks are called by external
  services that can't send bearer tokens)

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

src/db/libsql/routines.rs

@@ -530,10 +530,24 @@ impl RoutineStore for LibSqlBackend {
     async fn get_webhook_routine_by_path(
         &self,
         path: &str,
+        user_id: Option<&str>,
     ) -> Result<Option<Routine>, DatabaseError> {
         let conn = self.connect().await?;
-        let mut rows = conn
-            .query(
+        let mut rows = if let Some(uid) = user_id {
+            conn.query(
+                &format!(
+                    "SELECT {} FROM routines WHERE enabled = 1 AND trigger_type = 'webhook' \
+                     AND user_id = ?2 \
+                     AND (json_extract(trigger_config, '$.path') = ?1 \
+                     OR (json_extract(trigger_config, '$.path') IS NULL AND CAST(id AS TEXT) = ?1))",
+                    ROUTINE_COLUMNS
+                ),
+                params![path, uid],
+            )
+            .await
+            .map_err(|e| DatabaseError::Query(e.to_string()))?
+        } else {
+            conn.query(
                 &format!(
                     "SELECT {} FROM routines WHERE enabled = 1 AND trigger_type = 'webhook' \
                      AND (json_extract(trigger_config, '$.path') = ?1 \
@@ -543,7 +557,8 @@ impl RoutineStore for LibSqlBackend {
                 params![path],
             )
             .await
-            .map_err(|e| DatabaseError::Query(e.to_string()))?;
+            .map_err(|e| DatabaseError::Query(e.to_string()))?
+        };
 
         match rows
             .next()

src/db/mod.rs

@@ -545,6 +545,7 @@ pub trait RoutineStore: Send + Sync {
     async fn get_webhook_routine_by_path(
         &self,
         path: &str,
+        user_id: Option<&str>,
     ) -> Result<Option<Routine>, DatabaseError>;
 
     /// List routine runs that were dispatched as full_job but have not yet

src/db/postgres.rs

@@ -529,8 +529,9 @@ impl RoutineStore for PgBackend {
     async fn get_webhook_routine_by_path(
         &self,
         path: &str,
+        user_id: Option<&str>,
     ) -> Result<Option<Routine>, DatabaseError> {
-        self.store.get_webhook_routine_by_path(path).await
+        self.store.get_webhook_routine_by_path(path, user_id).await
     }
 
     async fn list_dispatched_routine_runs(&self) -> Result<Vec<RoutineRun>, DatabaseError> {