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/database/model_target.go

@@ -7,6 +7,7 @@ const (
 	TargetTypeHTTP     TargetType = "http"
 	TargetTypeDatabase TargetType = "database"
 	TargetTypeLog      TargetType = "log"
+	TargetTypeSlack    TargetType = "slack"
 )
 
 // Target represents a delivery target for a webhook