discover: normalize source_url trailing slash before insert

Same recipe URL with vs without trailing slash was producing duplicate
discover corpus rows because UNIQUE(source_url) is byte-exact:
  https://www.tasteofhome.com/recipes/falafel  → id 7
  https://www.tasteofhome.com/recipes/falafel/ → id 3 (manually pasted)

Caught 2026-05-02 when Alice pasted his first 4 with trailing slashes,
then a follow-up listing-page extractor stripped them, producing 1:1
dupes. rstrip('/') in insert_discovered_recipe normalizes at the
persistence layer so all callers get the dedup for free.

Existing data manually fixed: deleted dupes 7,8,5; stripped trailing
slashes off rows 3,4,6 to canonical form. Corpus now clean (4 rows).

cauldron/db.py

@@ -2251,7 +2251,18 @@ class DB:
     ) -> int | None:
         """INSERT a freshly-scraped recipe in 'raw' state. Returns the new
         row id, or None if the source_url was already present (UNIQUE
-        violation = duplicate scrape, treat as skip)."""
+        violation = duplicate scrape, treat as skip).
+
+        Normalizes source_url by stripping trailing slashes so that
+        `.../recipes/falafel` and `.../recipes/falafel/` map to the same
+        UNIQUE key. 2026-05-02: caught when manual `/discover` paste
+        included trailing slash but listing-page extractor stripped it,
+        producing 1:1 duplicates."""
+        # URL canonicalization — single rstrip is safe for recipe paths
+        # (they always have a non-slash terminal segment; `https://host/`
+        # alone wouldn't be a valid recipe URL anyway).
+        if source_url.endswith("/"):
+            source_url = source_url.rstrip("/")
         with self.conn() as c, c.cursor() as cur:
             cur.execute(
                 """INSERT IGNORE INTO cauldron_discovered_recipes