feat(notifications): add API routes for rules and history (PR 2 of #254)

[Pragati5-DEBUG]

This is PR 2 of 5 for #254 (notification channels for high/critical findings).

PR 1 added the database schema and Pydantic models. This PR adds the REST API to manage notification rules and read delivery history.

New endpoints:

GET /api/v1/notifications/rules — list all rules
POST /api/v1/notifications/rules — create a rule
GET /api/v1/notifications/rules/{id} — get one rule
PATCH /api/v1/notifications/rules/{id} — update a rule
DELETE /api/v1/notifications/rules/{id} — delete a rule
GET /api/v1/notifications/history — list delivery history (?rule_id=, ?limit=, ?offset=)
Also includes:

NotificationRuleUpdate model for partial PATCH updates
Webhook URL and email validation before saving
Integration tests for CRUD, validation, and history listing
Planned order (PR 2 of 5):

DB schema + models ✅ merged
API routes ← this PR
Notification service (webhook/email, dedupe, redaction)
Executor hook (trigger after scan)
Settings UI
No actual notification sending in this PR — routes and validation only.

Hi @utksh1 — continuing the split from #360. Please review when you can.

Related Issues
#254
Related to closed #360 (split into smaller PRs)

Type of Change
New feature (non-breaking change which adds functionality)

Documentation update
How Has This Been Tested?
.\venv\Scripts\python.exe -m pytest testing/backend/integration/test_notification_routes.py -v
4 integration tests pass
CRUD flow, invalid webhook rejection, email target, history list with rule_id filter
ruff check backend testing/backend passes
Checklist

My code follows the code style of this project.

I have performed a self-review of my own code.

My changes generate no new warnings.