fix: variations/adapt-recipe silent errors and no-op duplicates

AI adapt/variations flows had no catch on their fetch calls, so a
network failure surfaced as nothing happening rather than an error
toast. They also unconditionally persisted the AI's output even when
it was identical to the source recipe, and the adapt route never
recorded a recipeVariations row (only plain forks did) or enforced
the per-tier recipe-count limit other create paths already check.

v0.39.0

package.json

@@ -1,6 +1,6 @@
 {
   "name": "epicure",
-  "version": "0.38.0",
+  "version": "0.39.0",
   "private": true,
   "scripts": {
     "dev": "pnpm --filter web dev",