feat: add Slack target type for incoming webhook notifications

Add a new 'slack' target type that sends webhook events as formatted
messages to any Slack-compatible incoming webhook URL (Slack, Mattermost,
and other compatible services).

Messages include event metadata (method, content type, timestamp, body
size) and the payload pretty-printed in a code block. JSON payloads are
automatically formatted with indentation; non-JSON payloads are shown as
raw text. Large payloads are truncated at 3500 chars.

Changes:
- Add TargetTypeSlack constant to model_target.go
- Add SlackTargetConfig struct and deliverSlack method to delivery engine
- Add FormatSlackMessage (exported) for building Slack message text
- Route slack targets in processDelivery switch
- Handle slack type in HandleTargetCreate with webhook_url config
- Add slack option to source_detail.html target creation form
- Add comprehensive tests (config parsing, message formatting, delivery
  success/failure, routing)
- Update README with slack target documentation

internal/handlers/source_management.go

@@ -519,14 +519,14 @@ func (h *Handlers) HandleTargetCreate() http.HandlerFunc {
 
 		// Validate target type
 		switch targetType {
-		case database.TargetTypeHTTP, database.TargetTypeDatabase, database.TargetTypeLog:
+		case database.TargetTypeHTTP, database.TargetTypeDatabase, database.TargetTypeLog, database.TargetTypeSlack:
 			// valid
 		default:
 			http.Error(w, "Invalid target type", http.StatusBadRequest)
 			return
 		}
 
-		// Build config JSON for HTTP targets
+		// Build config JSON based on target type
 		var configJSON string
 		if targetType == database.TargetTypeHTTP {
 			if url == "" {
@@ -542,6 +542,20 @@ func (h *Handlers) HandleTargetCreate() http.HandlerFunc {
 				return
 			}
 			configJSON = string(configBytes)
+		} else if targetType == database.TargetTypeSlack {
+			if url == "" {
+				http.Error(w, "Webhook URL is required for Slack targets", http.StatusBadRequest)
+				return
+			}
+			cfg := map[string]interface{}{
+				"webhook_url": url,
+			}
+			configBytes, err := json.Marshal(cfg)
+			if err != nil {
+				http.Error(w, "Internal server error", http.StatusInternalServerError)
+				return
+			}
+			configJSON = string(configBytes)
 		}
 
 		maxRetries := 0 // default: fire-and-forget (no retries)