refactor: enforce non-null constraints on consume_coupon fields

[purplesmoke05]

📌 Description

Refactor indexed position aggregation to enforce token-type-specific query invariants in SQL, remove defensive assertions and broad tuple casts, and harden coupon consumption nullability.

✅ Related Issues

• Close [REFACTOR] Remove defensive asserts from position aggregation queries #1793

🔄 Changes

• Refactored /Position/{account_address} indexed aggregation to build token-type-specific queries and merge them with UNION ALL.
• Moved pagination and ordering for aggregated positions back to SQL to preserve existing behavior and avoid Python-side full-result pagination.
• Replaced broad result typing workarounds with Nullable(...) and type_coerce(...) where SQLAlchemy result typing needed explicit nullability/type hints.
• Removed defensive assert usage in aggregated position handling by making token cache presence guaranteed by the query shape.
• Enforced NOT NULL for consume_coupon.amount and consume_coupon.block_timestamp via migration and updated ORM typing accordingly.
• Simplified coupon consumption response handling to treat amount and block_timestamp as non-null.
• Added regression tests for missing token cache rows in aggregated position results.
• Added migration tests for cleanup of invalid consume_coupon rows and NOT NULL enforcement.

📌 Checklist

• I have added tests where necessary.
• I have updated the documentation where necessary.

[github-actions]

Coverage Report •

File	Stmts	Miss	Branch	BrPart	Cover	Missing
app/api/routers
position.py	460	96	150	27	79%	479, 486, 533–534, 558–563, 621–622, 649–650, 684–689, 729–730, 757–758, 792–797, 1069–1070, 1097–1098, 1141–1146, 1217–1228, 1230–1231, 1241, 1279–1290, 1292–1293, 1303, 1339–1340, 1384–1393, 1395–1396, 1405, 1441, 1478, 1513, 1542, 1605–1607, 1615, 1622, 1894, 1903
app/model/db
idx_consume_coupon.py	12	0	0	0	100%
tests/app
position_PositionCoupon_test.py	409	2	8	2	99%	190, 197
position_Position_test.py	460	2	24	2	99%	73, 82
TOTAL	37874	1704	3692	759	95%

Tests	Skipped	Failures	Errors	Time
1269	0 💤	0 ❌	0 🔥	10m 18s ⏱️