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

README.md

@@ -291,7 +291,7 @@ events should be forwarded.
 | `id`             | UUID       | Primary key |
 | `webhook_id`     | UUID       | Foreign key → Webhook |
 | `name`           | string     | Human-readable name |
-| `type`           | TargetType | One of: `http`, `database`, `log` |
+| `type`           | TargetType | One of: `http`, `slack`, `database`, `log` |
 | `active`         | boolean    | Whether deliveries are enabled (default: true) |
 | `config`         | JSON text  | Type-specific configuration |
 | `max_retries`    | integer    | Maximum retry attempts for HTTP targets (0 = fire-and-forget, >0 = retries with backoff) |
@@ -463,6 +463,16 @@ target simply marks the delivery as immediately successful. The
 per-webhook DB IS the dedicated event database — that's the whole point
 of the database target type.
 
+The **Slack target type** sends webhook events as formatted messages to
+any Slack-compatible incoming webhook URL (works with Slack, Mattermost,
+and other compatible services). Each message includes event metadata
+(HTTP method, content type, timestamp, body size) and the payload
+pretty-printed in a code block. JSON payloads are automatically
+formatted with indentation for readability; non-JSON payloads are shown
+as raw text. Large payloads are truncated to keep messages reasonable.
+Config stores `webhook_url` — the Slack/Mattermost incoming webhook
+endpoint.
+
 The database uses the
 [modernc.org/sqlite](https://pkg.go.dev/modernc.org/sqlite) driver at
 runtime, though CGO is required at build time due to the transitive
@@ -601,7 +611,7 @@ fine — startup recovery rescans the database anyway).
 
 **Scope:** Circuit breakers only apply to **HTTP targets with
 `max_retries` > 0**. Fire-and-forget HTTP targets (`max_retries` == 0),
-database targets (local operations), and log targets (stdout) do not use
+Slack targets, database targets (local operations), and log targets (stdout) do not use
 circuit breakers.
 
 When a circuit is open and a new delivery arrives, the engine marks the
@@ -860,6 +870,8 @@ linted, tested, and compiled.
       retries with exponential backoff when max_retries>0)
 - [x] Implement database target type (store events in per-webhook DB)
 - [x] Implement log target type (console output)
+- [x] Implement Slack target type (Slack/Mattermost incoming webhook
+      notifications with pretty-printed payloads)
 - [x] Webhook management pages (list, create, edit, delete)
 - [x] Webhook request log viewer with pagination
 - [x] Entrypoint and target management UI