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.

pkg/config/config.go

@@ -15,7 +15,7 @@
 //
 // Usage:
 //
-//	import "git.eeqj.de/sneak/webhooker/pkg/config"
+//	import "sneak.berlin/go/webhooker/pkg/config"
 //
 //	// Set the environment explicitly
 //	config.SetEnvironment("prod")