feat: M10 — agent metadata collection, Apache httpd + HAProxy target connectors

Agents now report OS, architecture, IP address, hostname, and version
via heartbeat using runtime.GOOS, runtime.GOARCH, and net.Dial. New
migration adds columns to agents table. Heartbeat handler, service,
and repository updated to accept and persist metadata. GUI shows
OS/Arch in agent list and full system info in agent detail page.

Apache httpd connector: separate cert/chain/key files, apachectl
configtest validation, graceful reload. HAProxy connector: combined
PEM file (cert+chain+key), optional config validation, reload.
Both wired into agent binary's target connector switch.

14 tests for new connectors. All existing tests updated for new
Heartbeat/UpdateHeartbeat signatures. Docs updated across README,
architecture, concepts, and connectors guides.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/domain/connector.go

@@ -37,6 +37,19 @@ type Agent struct {
 	LastHeartbeatAt *time.Time  `json:"last_heartbeat_at,omitempty"`
 	RegisteredAt    time.Time   `json:"registered_at"`
 	APIKeyHash      string      `json:"api_key_hash"`
+	OS              string      `json:"os"`
+	Architecture    string      `json:"architecture"`
+	IPAddress       string      `json:"ip_address"`
+	Version         string      `json:"version"`
+}
+
+// AgentMetadata contains runtime metadata reported by agents via heartbeat.
+type AgentMetadata struct {
+	OS           string `json:"os"`
+	Architecture string `json:"architecture"`
+	Hostname     string `json:"hostname"`
+	IPAddress    string `json:"ip_address"`
+	Version      string `json:"version"`
 }
 
 // AgentStatus represents the operational status of an agent.
@@ -60,7 +73,9 @@ const (
 type TargetType string
 
 const (
-	TargetTypeNGINX TargetType = "NGINX"
-	TargetTypeF5    TargetType = "F5"
-	TargetTypeIIS   TargetType = "IIS"
+	TargetTypeNGINX   TargetType = "NGINX"
+	TargetTypeApache  TargetType = "Apache"
+	TargetTypeHAProxy TargetType = "HAProxy"
+	TargetTypeF5      TargetType = "F5"
+	TargetTypeIIS     TargetType = "IIS"
 )