Research federation issue fix and document findings

Cargo.toml

@@ -72,6 +72,7 @@ urlencoding = "2.1"
 ammonia = "4.0"
 regex = "1.10"
 dotenvy = "0.15"
+sha2 = "0.10"
 
 [dev-dependencies]
 tokio-test = "0.4"

migrations/006_add_content_hash.sql

@@ -0,0 +1,5 @@
+-- Add content hash column for deduplication
+ALTER TABLE recipes ADD COLUMN content_hash TEXT;
+
+-- Index for fast duplicate lookups
+CREATE INDEX idx_recipes_content_hash ON recipes(content_hash);