Only force-refresh all snapshots on manual trigger

Scheduled runs now only refresh snapshots older than 24h (the
existing default behavior of refresh-snapshots.js), and
workflow_dispatch gains a force_all input to refresh everything
on demand from the GitHub UI.

Refs #22

Author: claude

.github/workflows/refresh-snapshots.yml

@@ -5,7 +5,12 @@ name: Refresh snapshots
 on:
   schedule:
     - cron: "0 6 * * *" # daily at 06:00 UTC
-  workflow_dispatch: {}
+  workflow_dispatch:
+    inputs:
+      force_all:
+        description: "Force-refresh all snapshots, even ones less than 24h old"
+        type: boolean
+        default: false
 
 permissions:
   contents: write
@@ -24,9 +29,11 @@ jobs:
       - name: Install dependencies
         run: npm ci
 
-      # Force a fresh fetch of every snapshot regardless of age.
-      - name: Refresh all HTML snapshots
-        run: npm run refresh -- --force
+      # Scheduled runs only refresh snapshots older than 24h. Manually
+      # triggered runs can opt into refreshing every snapshot via the
+      # "force_all" input.
+      - name: Refresh HTML snapshots
+        run: npm run refresh -- ${{ inputs.force_all && '--force' || '' }}
 
       # Run the live assertions against the freshly fetched pages, so a site
       # that changed its markup turns this scheduled run red.