rework: migrate module path, fix healthcheck URL, add architecture notes

- Migrate Go module path from git.eeqj.de/sneak/webhooker to
  sneak.berlin/go/webhooker (go.mod, pkg/config/go.mod, all imports)
- Drop .json extension from healthcheck endpoint: /.well-known/healthcheck
  (routes.go, Dockerfile HEALTHCHECK, README)
- Add Rate Limiting section to README Design: global rate limiting must
  not apply to webhook endpoints; per-webhook configurable limits instead
- Add Database Architecture section to README Design: separate SQLite
  files for main app config vs per-processor data (input logs, processor
  logs, output queues)

Addresses review feedback from sneak on PR #6.

go.mod

@@ -1,11 +1,10 @@
-module git.eeqj.de/sneak/webhooker
+module sneak.berlin/go/webhooker
 
 go 1.23.0
 
 toolchain go1.24.1
 
 require (
-	git.eeqj.de/sneak/webhooker/pkg/config v0.0.0-00010101000000-000000000000
 	github.com/99designs/basicauth-go v0.0.0-20230316000542-bf6f9cbbf0f8
 	github.com/getsentry/sentry-go v0.25.0
 	github.com/go-chi/chi v1.5.5
@@ -22,6 +21,7 @@ require (
 	gorm.io/driver/sqlite v1.5.4
 	gorm.io/gorm v1.25.5
 	modernc.org/sqlite v1.28.0
+	sneak.berlin/go/webhooker/pkg/config v0.0.0-00010101000000-000000000000
 )
 
 require (
@@ -85,4 +85,4 @@ require (
 	modernc.org/token v1.0.1 // indirect
 )
 
-replace git.eeqj.de/sneak/webhooker/pkg/config => ./pkg/config
+replace sneak.berlin/go/webhooker/pkg/config => ./pkg/config