feat: add Slack target type for incoming webhook notifications (#47)

## Summary

Adds 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).

closes #44

## What it does

When a webhook event is received, the Slack target:

1. Formats a human-readable message with event metadata (HTTP method, content type, timestamp, body size)
2. Pretty-prints the payload in a code block — JSON payloads get indented formatting, non-JSON payloads are shown as raw text
3. Truncates large payloads at 3500 characters to keep Slack messages reasonable
4. POSTs the message as a `{"text": "..."}` JSON payload to the configured webhook URL

## Changes

- **`internal/database/model_target.go`** — Add `TargetTypeSlack` constant
- **`internal/delivery/engine.go`** — Add `SlackTargetConfig` struct, `deliverSlack` method, `FormatSlackMessage` function (exported), `parseSlackConfig` helper. Route slack targets in `processDelivery` switch.
- **`internal/handlers/source_management.go`** — Handle `slack` type in `HandleTargetCreate`, building `webhook_url` config from the URL form field
- **`templates/source_detail.html`** — Add "Slack" option to target type dropdown with URL field and helper text
- **`README.md`** — Document the new target type, update roadmap

## Tests

- `TestParseSlackConfig_Valid` / `_Empty` / `_MissingWebhookURL` — Config parsing
- `TestFormatSlackMessage_JSONBody` / `_NonJSONBody` / `_EmptyBody` / `_LargeJSONTruncated` — Message formatting
- `TestDeliverSlack_Success` / `_Failure` / `_InvalidConfig` — End-to-end delivery
- `TestProcessDelivery_RoutesToSlack` — Routing from processDelivery switch

All existing tests continue to pass. `docker build .` (which runs `make check`) passes clean.

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #47
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

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
@@ -601,8 +611,8 @@ 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
-circuit breakers.
+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
 delivery as `retrying` and schedules a retry timer for after 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