feat(webhooks): add outgoing webhook dispatcher with retries and HMAC signing

[Alimzy]

Adds webhook persistence and dispatch flow for plan lifecycle events:
plan.created
plan.pinged
plan.claimable
Adds PostgreSQL schema:
webhook_endpoints
webhook_dispatches
Implements background dispatcher with:
retry loop up to 5 attempts
exponential backoff
HMAC-SHA256 payload signing via X-Signature
status persistence for success/failure and next attempt scheduling
Wires dispatcher startup in main.rs
Enqueues webhook events from:
api.rs
inactivity_watchdog.rs
Closes #826

[drips-wave]

@Alimzy Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[ONEONUORA]

@Alimzy
un cargo fmt --all -- --check

and clippy

[ONEONUORA]

@Alimzy
Run cargo fmt --all -- --check

and clippy

[ONEONUORA]

@Alimzy
Run cargo fmt --all -- --check

[ONEONUORA]

@Alimzy
Run cargo fmt --all -- --check

[ONEONUORA]

@Alimzy
Pls fix conflict

[ONEONUORA]

Run cargo fmt --all -- --check
error: unexpected closing delimiter: }
--> /home/runner/work/InheritX/InheritX/backend/src/api.rs:419:1
|
221 | ) -> impl IntoResponse {
| - this opening brace...
...
418 | }
| - ...matches this closing brace
419 | }
| ^ unexpected closing delimiter

Error writing files: failed to resolve mod api: cannot parse /home/runner/work/InheritX/InheritX/backend/src/api.rs
Error: Process completed with exit code 1.

@Alimzy

[ONEONUORA]

Run cargo fmt --all -- --check

[ONEONUORA]

@Alimzy
Update on this Pr
you will be unassign in 6hrs