fix(purchases): default extra_amount to 0 when price_history is missing

If no track_price_history / album_price_history row applies to a
purchase, the view currently returns extra_amount = sp.amount because
the COALESCE falls back to 0 and (sp.amount - 0) = sp.amount.

Cases this hits:
- Content deleted before its price was ever indexed
- Backfilled historical purchases whose content predates
  price_history tracking
- Test environments that don't fixture price_history

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."
Production purchases via the Go indexer always have price_history
coverage (the indexer validates against it), so this only affects the
fallback path.

This was a follow-up commit on the PR #815 branch that didn't make it
into the squash-merge.

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,