Refactor architecture: extract reconciler, add interfaces, reduce complexity, improve test coverage

Architecture:
- Extract reconcileNetworking into internal/reconcile package with 7 consumer-side interfaces
- Add locked modifyState helper to fix state.yaml read-modify-write race condition
- Extract CrowdSec and VPN handler groups with interfaces documenting dependency surface
- Replace raw map[string]any YAML manipulation with typed AddDHCPStaticLease/RemoveDHCPStaticLease
- Extract Cloudflare API functions into cfClient struct, eliminating repeated auth boilerplate

Complexity reduction:
- haproxy.GenerateWithOpts: 50 → 6 (extracted 8 focused helpers)
- reconcile.Reconcile: 42 → 11 (extracted buildRoutes, buildDNSEntries, writeHAProxyConfig)
- AutheliaUpdateConfig: 25 → 10 (extracted enableAuthelia/disableAuthelia)

Test coverage improvements:
- reconcile: 4.5% → 56.8% (stub-based tests for route building, DNS entries, orchestration)
- dnsfilter: 10.7% → 45.6% (Manager.Compile, AddList, ToggleList, custom entries)
- config: 67.3% → 89.8% (DHCP static lease mutation tests)

internal/api/v1/response.go

@@ -10,9 +10,9 @@ import (
 // Error responses use the Error field.
 // Message-only responses use the Message field.
 type APIResponse struct {
-	Data    any `json:"data,omitempty"`
-	Message string      `json:"message,omitempty"`
-	Error   string      `json:"error,omitempty"`
+	Data    any    `json:"data,omitempty"`
+	Message string `json:"message,omitempty"`
+	Error   string `json:"error,omitempty"`
 }
 
 // respondJSON writes a JSON response with the given status code.
@@ -33,4 +33,3 @@ func respondMessage(w http.ResponseWriter, status int, message string) {
 func respondError(w http.ResponseWriter, status int, message string) {
 	respondJSON(w, status, APIResponse{Error: message})
 }
-