Add S-expression recipe support

- Add format detection that correctly handles ; comments
- Import artdag.sexp parser/compiler with YAML fallback
- Add execute_step_sexp and run_plan_sexp Celery tasks
- Update recipe upload to handle both S-expr and YAML formats

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tasks/__init__.py

@@ -4,10 +4,15 @@
 # 1. analyze_input - Extract features from input media
 # 2. execute_step - Execute a single step from the plan
 # 3. run_plan - Orchestrate execution of a full plan
+#
+# S-expression tasks:
+# 4. execute_step_sexp - Execute step from S-expression
+# 5. run_plan_sexp - Run plan from S-expression
 
 from .analyze import analyze_input, analyze_inputs
 from .execute import execute_step
 from .orchestrate import run_plan, run_recipe
+from .execute_sexp import execute_step_sexp, run_plan_sexp
 
 __all__ = [
     "analyze_input",
@@ -15,4 +20,7 @@ __all__ = [
     "execute_step",
     "run_plan",
     "run_recipe",
+    # S-expression tasks
+    "execute_step_sexp",
+    "run_plan_sexp",
 ]