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>

templates/source_detail.html

@@ -92,17 +92,22 @@
                         <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>
                     </div>
                     <div x-show="targetType === 'http'">
-                        <input type="url" name="url" placeholder="https://example.com/webhook" class="input text-sm">
+                        <input type="url" name="url" placeholder="https://example.com/webhook" :disabled="targetType !== 'http'" class="input text-sm">
                     </div>
                     <div x-show="targetType === 'http'" class="flex gap-2 items-center">
                         <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/..." :disabled="targetType !== 'slack'" 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>