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

templates/source_detail.html

@@ -92,6 +92,7 @@
                         <input type="text" name="name" placeholder="Target name" required class="input text-sm flex-1">
                         <select name="type" x-model="targetType" class="input text-sm w-32">
                             <option value="http">HTTP</option>
+                            <option value="slack">Slack</option>
                             <option value="database">Database</option>
                             <option value="log">Log</option>
                         </select>
@@ -103,6 +104,10 @@
                         <label class="text-sm text-gray-700">Max retries (0 = fire-and-forget):</label>
                         <input type="number" name="max_retries" value="0" min="0" max="20" class="input text-sm w-24">
                     </div>
+                    <div x-show="targetType === 'slack'">
+                        <input type="url" name="url" placeholder="https://hooks.slack.com/services/..." class="input text-sm">
+                        <p class="text-xs text-gray-500 mt-1">Slack or Mattermost incoming webhook URL. Payloads are pretty-printed in code blocks.</p>
+                    </div>
                     <button type="submit" class="btn-primary text-sm">Add Target</button>
                 </form>
             </div>