In-site feedback reply (plain-text v1)

Reply to a reader from the admin inbox instead of a mailto. Per Codex: keep v1
plain text (no rich editor — defers the user's bold/bullets ask as a fast-follow).

* DB: feedback_replies table (feedback_id, user_id, message, sent_to, sent_at),
  created on the live DB.
* email_send.send_feedback_reply: plain-text "Re: Your Upbeat Bytes feedback"
  with a quoted context block, no analytics/account details.
* API: POST /api/admin/feedback/{id}/reply — admin-gated, requires the feedback
  exists (404) and has a contact_email (400), trims+caps the message; sends via
  SMTP and only records the reply on success (502 on send failure so the UI keeps
  the draft); marks the item read. Feedback list now includes each item's replies.
* Frontend: inline composer (Send/Cancel, sending state, error keeps draft) +
  reply thread under the message; Reply only shows when there's an address,
  else "No reply address".

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

goodnews/db.py

@@ -240,6 +240,16 @@ CREATE TABLE IF NOT EXISTS feedback (
     read_at TEXT
 );
 CREATE INDEX IF NOT EXISTS idx_feedback_created ON feedback(created_at);
+
+CREATE TABLE IF NOT EXISTS feedback_replies (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    feedback_id INTEGER NOT NULL REFERENCES feedback(id) ON DELETE CASCADE,
+    user_id INTEGER REFERENCES users(id) ON DELETE SET NULL,
+    message TEXT NOT NULL,
+    sent_to TEXT NOT NULL,
+    sent_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+CREATE INDEX IF NOT EXISTS idx_feedback_replies_fid ON feedback_replies(feedback_id);
 """