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>

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