refactor: enforce non-null constraints on notification fields

[purplesmoke05]

📌 Description

This PR updates notification nullability for issue #1794.

Fields that should always exist are aligned across the migration, ORM model, and API schema. address stays nullable because some notification types do not have an address.

✅ Related Issues

• Close [REFACTOR] Normalize notification nullability in API and ORM models #1794

🔄 Changes

• Added a migration to make these notification fields NOT NULL:
  • notification_type
  • priority
  • is_read
  • is_flagged
  • is_deleted
  • block_timestamp
  • metainfo
• Backfilled priority with 0 and boolean flags with False
• Deleted invalid notification rows with missing notification_type, block_timestamp, or metainfo

📌 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
notification.py	106	9	44	7	91%	101, 105, 147, 172, 212, 247, 301, 306, 357
app/model/db
notification.py	72	4	6	3	94%	42, 53, 132, 141
app/model/schema
notification.py	78	0	0	0	100%
tests/app
notification_Notifications_GET_test.py	154	0	0	0	100%
TOTAL	37855	1671	3688	757	95%

Tests	Skipped	Failures	Errors	Time
1269	0 💤	0 ❌	0 🔥	10m 11s ⏱️

[Copilot]

Pull request overview

This PR enforces non-null guarantees for persisted notification records by aligning DB constraints, ORM typing, and API schema expectations (issue #1794), while keeping address/account_address nullable for notification types that don’t have an address.

Changes:

• Added an Alembic migration to backfill defaults, delete invalid rows, and set several notification columns to NOT NULL.
• Updated the SQLAlchemy Notification model and schema typings to reflect non-null fields, and allowed account_address to be nullable in the API schema.
• Updated API router serialization/type-check scaffolding and expanded migration/API tests to cover the new constraints and nullable account_address.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
migrations/versions/93f1494c3975_v26_6_0_feature_1794.py	Backfills/deletes invalid notifications and applies NOT NULL constraints (incl. JSON-null handling for metainfo).
app/model/db/notification.py	Makes several notification columns non-nullable and centralizes datetime formatting used by API serialization.
app/model/schema/notification.py	Updates response typing to require non-null fields and to allow account_address to be null.
app/api/routers/notification.py	Simplifies notification serialization and removes redundant null assertions now covered by DB/model guarantees.
docs/ibet_wallet_api.yaml	Updates OpenAPI schema to allow account_address: null.
tests/migrations/upgrade_test.py	Extends migration upgrade test to validate the new notification nullability/backfill/delete behavior.
tests/app/notification_Notifications_GET_test.py	Adds coverage for a notification with account_address/address as None and updates expected counts.

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