Fail deliveries on archive errors; validate expiry at creation (#43)

Two review findings on the database archiving target:

- An archive error now records the attempt as failed with the
  error string and marks the delivery failed, instead of logging
  the error and reporting success. A target that could not do its
  one job must not claim it did.
- The archive expiry is now actually configurable: the add-target
  form gains an expiry field for database targets, and the value
  is validated at creation time via the new
  delivery.ValidateArchiveExpiry (empty, "never", or a positive
  Go duration), rejecting bad values with a 400 at the only place
  a human can fix them, mirroring how Slack target URLs are
  validated at creation.

Test updates: a forced archive failure asserts a failed delivery
with a recorded error and no archive file; config builder tests
cover empty/never/duration and rejection paths; the two engine
tests that exercise the database target now build engines with a
real webhook DB manager since archiving is no longer a no-op; the
reopen-debounce test uses a wider window so parallel test load
cannot make two rapid writes straddle it.

templates/source_detail.html

@@ -113,6 +113,10 @@
                         <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>
+                    <div x-show="targetType === 'database'">
+                        <input type="text" name="expiry" placeholder="never" :disabled="targetType !== 'database'" class="input text-sm">
+                        <p class="text-xs text-gray-500 mt-1">Archive expiry: "never" (default) keeps rows forever, or a duration like "720h" prunes older rows.</p>
+                    </div>
                     <button type="submit" class="btn-primary text-sm">Add Target</button>
                 </form>
             </div>