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.

Dockerfile

@@ -64,6 +64,6 @@ USER webhooker
 EXPOSE 8080
 
 HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
-    CMD wget --no-verbose --tries=1 --spider http://localhost:8080/.well-known/healthcheck.json || exit 1
+    CMD wget --no-verbose --tries=1 --spider http://localhost:8080/.well-known/healthcheck || exit 1
 
 CMD ["./webhooker"]