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.

internal/server/routes.go

@@ -4,11 +4,11 @@ import (
 	"net/http"
 	"time"
 
-	"git.eeqj.de/sneak/webhooker/static"
 	sentryhttp "github.com/getsentry/sentry-go/http"
 	"github.com/go-chi/chi"
 	"github.com/go-chi/chi/middleware"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
+	"sneak.berlin/go/webhooker/static"
 )
 
 func (s *Server) SetupRoutes() {
@@ -63,7 +63,7 @@ func (s *Server) SetupRoutes() {
 	})
 
 	s.router.Get(
-		"/.well-known/healthcheck.json",
+		"/.well-known/healthcheck",
 		s.h.HandleHealthCheck(),
 	)