Default v_usdc_purchases.extra_amount to 0 when price_history is missing

If no track_price_history / album_price_history row applies to a
purchase (e.g., content was deleted before its price was indexed, a
backfilled historical purchase whose price predates the price_history
table, or tests not seeding price_history), the view currently returns
extra_amount = sp.amount because COALESCE falls back to 0 and
(sp.amount - 0) = sp.amount.

Change the fallback to sp.amount so the subtraction nets to 0 —
matches "we don't know the base price, don't claim there was a tip."

This was the third commit on the PR #815 branch that didn't make it
into the squash-merge (alongside the updated_at fix in d578302).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: rickyrombo

ddl/views/v_usdc_purchases.sql

@@ -34,7 +34,7 @@ SELECT
                  LIMIT 1
               )
             END,
-            0
+            sp.amount  -- no price_history match -> treat full amount as base price (extra_amount = 0)
         ),
         0
     ) AS extra_amount,

sql/01_schema.sql

@@ -9644,7 +9644,7 @@ CREATE VIEW public.v_usdc_purchases AS
               WHERE ((aph.playlist_id = sp.content_id) AND (aph.block_timestamp <= sp.created_at))
               ORDER BY aph.block_timestamp DESC
              LIMIT 1)
-        END, (0)::bigint)), (0)::bigint) AS extra_amount,
+        END, sp.amount)), (0)::bigint) AS extra_amount,
     (sp.access_type)::public.usdc_purchase_access_type AS access,
     sp.city,
     sp.region,