Swap purchase readers from usdc_purchases to v_usdc_purchases

[rickyrombo]

Summary

Step 2 of the purchases-domain cutover. Depends on #815 (backfill + view + trigger).

All ~17 Go API routes that joined usdc_purchases swap to v_usdc_purchases. Once #815 lands first, this PR's base auto-updates to main and only this diff remains.

Code changes are mostly one-token table-name renames in route SQL — the view absorbs schema differences. Test fixtures are rewritten to seed sol_purchases + sol_payments instead of usdc_purchases.

Route changes (17 files)

v_usdc_purchases swapped in for usdc_purchases in:

• api/v1_users_purchases.go, _count.go, _download.go
• api/v1_users_sales.go, _count.go, _aggregate.go, _download.go
• api/v1_users_purchasers.go, _count.go
• api/v1_explore_best_selling.go
• api/v1_users_library_tracks.go, v1_users_library_playlists.go
• api/comms_blasts.go, api/comms/chat.go, api/comms/validator.go
• api/dbv1/access.go

Test fixture changes (15 files)

Each test that fixtured usdc_purchases now seeds sol_purchases + (when splits matter) sol_payments + (when extra_amount matters) track_price_history/album_price_history. Dropped columns the view derives (seller_user_id, extra_amount, splits, access).

For tests that assert on splits[*].user_id, the seller's users.spl_usdc_payout_wallet is set to match the corresponding sol_payments.to_account so the view's resolution chain produces the expected user_id.

The global testdata.UsdcPurchasesFixtures (consumed by testAppWithFixtures) is replaced with SolPurchasesFixtures + SolPaymentsFixtures, and server_test.go:105 seeds both.

database/seed.go gains baseRow entries for sol_purchases, sol_payments, and album_price_history.

Test plan

• go build ./... clean (verified locally)
• go vet ./... clean (verified locally)
• go test ./api/... green against a running test postgres
• Spot-check a prod replica: SELECT * FROM v_usdc_purchases LIMIT 10 returns plausible rows after Backfill sol_purchases from usdc_purchases #815 is deployed
• Smoke-check the affected endpoints in dev:
  • /v1/users/{id}/purchases, /sales, /purchasers, /sales/aggregate
  • /v1/users/{id}/purchases/download/{json,csv}, /sales/download/{json,csv}
  • /v1/explore/best-selling
  • /v1/full/users/{id}/library/tracks?type=purchase, /library/{albums,playlists}?type=purchase
  • Chat blasts with customer_audience

Not in this PR

• Python index_payment_router keeps writing usdc_purchases (the legacy notification trigger also keeps firing, dedupes via shared group_id with the new trigger from Backfill sol_purchases from usdc_purchases #815).
• vendor is dropped from notifications (frontends must tolerate null).
• seller_user_id is now the current content owner, not snapshotted at purchase time. Acceptable per design discussion.

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