feat(porch-store): add digest column to IdempotencyKey table

- Add digest field to store sha256(method|path|body) separately from key
- Enables detection of "same key, different body" in future tasks
- Update idempotent.wo insert to compute and store digest value
- Typechecker passes: exit 0

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
(cherry picked from commit 3a9bddcd1e3c11f5b371ce54cafc373685ca08b6)

docs/examples/porch/middleware/idempotent.wo

@@ -82,7 +82,11 @@ pub class Idempotent {
     let resp_json = json.encode(stored);
 
     let now = time.ticks();
-    try insert IdempotencyKey { key: key, response: resp_json, created_at: now } catch (e) nil;
+    -- Compute digest of method|path|body
+    let digest_input = "${req.method}|${req.path}|${req.body}";
+    let digest = sha256(bytes_of_text(digest_input));
+    let digest_hex = base64_encode(bytes_slice(digest, 0, 16));
+    try insert IdempotencyKey { key: key, response: resp_json, created_at: now, digest: digest_hex } catch (e) nil;
   }
 }
 

docs/examples/porch/middleware/store.wo

@@ -21,4 +21,5 @@ class IdempotencyKey {
   key:        Text @unique
   response:   Text
   created_at: Int
+  digest:     Text
 }