feat: add sender blocklist with priority matching and quick-add dropdown

- Add `blocked_senders` field to FeedConfig (alongside existing `allowed_senders`)
- Refactor sender matching to priority-based logic: exact block > exact allow > domain block > domain allow, enabling exceptions (e.g. allow toto@gmail.com despite blocking gmail.com)
- Add `POST /admin/feeds/:feedId/sender-filter` endpoint for quick allow/block from email detail view; returns 409 on conflict with opposite list
- Add ⋮ dropdown on From field in email detail with 4 options (allow/block sender/domain), inline success/error feedback
- Add blocked_senders textarea to create/edit feed forms
- 209 tests passing

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/routes/admin.tsx

@@ -380,6 +380,22 @@ app.get("/", async (c) => {
               </small>
             </div>
 
+            <div class="form-group">
+              <label for="blocked_senders">
+                Blocked senders (optional, one email or domain per line)
+              </label>
+              <textarea
+                id="blocked_senders"
+                name="blocked_senders"
+                rows={3}
+                placeholder={"spam@example.com\nunwanted.com"}
+              ></textarea>
+              <small>
+                Emails from these senders/domains are always rejected, even if
+                they match the allowlist.
+              </small>
+            </div>
+
             <input type="hidden" id="language" name="language" value="en" />
             <input type="hidden" name="view" value={view} />