nil pointer exception when uninstalled repo webhook is fired

[motatoes]

If an app installation fails or does not redirect to callback then github does not have a reflection of records for github app installation link or a repo records. This leads to a nil pointer exception

Excepted behaviour: digger should gracefully fail and comment with a message saying that there was no record found for the installation and asking the user to double check that the app was installed successfully and lead to the callback

time=2025-07-29T15:29:39.955Z level=INFO msg="Query OK" db.duration=4.264368ms db.rows=0 db.file=storage.go:564 db.query="SELECT * FROM \"github_app_installation_links\" WHERE (github_installation_id = 69477001 AND status=1) AND \"github_app_installation_links\".\"deleted_at\" IS NULL"
time=2025-07-29T15:29:39.955Z level=ERROR msg="Recovered from panic in handleIssueCommentEvent" error="runtime error: invalid memory address or nil pointer dereference"
Stack trace:
goroutine 148 [running]:
runtime/debug.Stack()
/usr/local/go/src/runtime/debug/stack.go:26 +0x5e
github.com/diggerhq/digger/backend/controllers.handleIssueCommentEvent.func1()
/go/src/github.com/diggerhq/digger/backend/controllers/github.go:1221 +0x3f
panic({0x26766e0?, 0x49e9e50?})
/usr/local/go/src/runtime/panic.go:787 +0x132
github.com/diggerhq/digger/backend/controllers.handleIssueCommentEvent({0x3243a00, 0x4c5a560}, 0xc000d0e4c0, {0x3214200, 0x4c5a560}, 0x14c054, {0x4c5a560, 0x0, 0x0?})
/go/src/github.com/diggerhq/digger/backend/controllers/github.go:1271 +0x6fe
created by github.com/diggerhq/digger/backend/controllers.DiggerController.GithubAppWebHook in goroutine 146
/go/src/github.com/diggerhq/digger/backend/controllers/github.go:124 +0xbe5
time=2025-07-29T15:29:48.050Z level=INFO msg="Incoming request" http.request.time=2025-07-29T15:29:48.050Z http.request.method=GET http.request.host=10.0.2.67:3000 http.request.path=/health http.request.query="" http.request.params=map[] http.request.route=/health http.request.ip=10.0.3.112 http.request.referer="" http.request.length=0 http.response.time=2025-07-29T15:29:48.050Z http.response.latency=59.792µs http.response.status=200 http.response.length=93 http.id=580d4e2d-cce5-4b6f-88e6-9908b6e4c630
[GIN] 2025/07/29 - 15:29:48 | 200 | 136.242µs | 10.0.3.112 | GET "/health"
time=2025-07-29T15:30:08.050Z level=INFO msg="Incoming request" http.request.time=2025-07-29T15:30:08.050Z http.request.method=GET http.request.host=10.0.2.67:3000 http.request.path=/health http.request.query="" http.request.params=map[] http.request.route=/health http.request.ip=10.0.3.112 http.request.referer="" http.request.length=0 http.response.time=2025-07-29T15:30:08.050Z http.response.latency=69.28µs http.response.status=200 http.response.length=93 http.id=c27fbf23-0902-4e00-b15d-5bce7a6b80fc