Initial commit: full Phases 1-6 implementation

Post-repair hardware validation pipeline for Proxmox cluster hosts.
Go orchestrator + in-image agent + mkosi live image + bundled dnsmasq
PXE + SQLite + HTMX/SSE UI + notify registry + janitor + full docs.

internal/notify/discord.go

@@ -0,0 +1,87 @@
+package notify
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+	"time"
+)
+
+// DiscordNotifier posts to a Discord incoming webhook. Body is rendered
+// as a single embed so Discord shows a colored sidebar matching event
+// severity. Discord rejects empty content+embeds; we always include the
+// embed so that never happens.
+type DiscordNotifier struct {
+	NameStr    string
+	WebhookURL string
+	HTTP       *http.Client
+}
+
+func NewDiscord(name, webhookURL string) *DiscordNotifier {
+	return &DiscordNotifier{
+		NameStr:    name,
+		WebhookURL: webhookURL,
+		HTTP:       &http.Client{Timeout: 10 * time.Second},
+	}
+}
+
+func (d *DiscordNotifier) Name() string { return d.NameStr }
+
+type discordPayload struct {
+	Embeds []discordEmbed `json:"embeds"`
+}
+
+type discordEmbed struct {
+	Title       string `json:"title,omitempty"`
+	Description string `json:"description,omitempty"`
+	URL         string `json:"url,omitempty"`
+	Color       int    `json:"color,omitempty"`
+}
+
+func (d *DiscordNotifier) Send(ctx context.Context, ev Event) error {
+	if d.WebhookURL == "" {
+		return fmt.Errorf("discord: no webhook_url configured")
+	}
+	payload := discordPayload{Embeds: []discordEmbed{{
+		Title:       ev.Title,
+		Description: ev.Body,
+		URL:         ev.URL,
+		Color:       discordColor(ev.Severity),
+	}}}
+	buf, err := json.Marshal(payload)
+	if err != nil {
+		return err
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, d.WebhookURL, bytes.NewReader(buf))
+	if err != nil {
+		return err
+	}
+	req.Header.Set("Content-Type", "application/json")
+	resp, err := d.HTTP.Do(req)
+	if err != nil {
+		return err
+	}
+	defer func() { _ = resp.Body.Close() }()
+	if resp.StatusCode >= 300 {
+		b, _ := io.ReadAll(resp.Body)
+		return fmt.Errorf("discord: %d: %s", resp.StatusCode, strings.TrimSpace(string(b)))
+	}
+	return nil
+}
+
+// discordColor returns the embed sidebar color for each severity.
+// Values are standard Discord decimal color codes.
+func discordColor(s Severity) int {
+	switch s {
+	case SeverityCritical:
+		return 0xE74C3C // red
+	case SeverityWarning:
+		return 0xF1C40F // yellow
+	default:
+		return 0x2ECC71 // green
+	}
+}