Add "truly done" ticket archiving with auto-unhide

- Let the user mark closed tickets as really done and hide them, so
  tickets closed prematurely by an agent stay visible until confirmed
- User-side "archived" flag stored in the DB (independent of Zammad),
  added via a schema migration on existing databases
- Hide archived tickets from the overview by default with an
  "erledigte einblenden" toggle; archive/unarchive from the ticket
  detail page and via a per-row quick action
- Poller automatically un-hides an archived ticket when new activity is
  detected, so nothing is missed

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/db.py

@@ -31,7 +31,8 @@ CREATE TABLE IF NOT EXISTS tickets (
     last_close_at           TEXT,
     raw_json                TEXT,
     first_seen_at           TEXT,
-    last_synced_at          TEXT
+    last_synced_at          TEXT,
+    archived                INTEGER NOT NULL DEFAULT 0
 );
 
 CREATE TABLE IF NOT EXISTS changes (
@@ -103,3 +104,11 @@ def get_conn():
 def init_db() -> None:
     with get_conn() as conn:
         conn.executescript(SCHEMA)
+        _migrate(conn)
+
+
+def _migrate(conn) -> None:
+    """Nachträgliche Schema-Anpassungen für bestehende Datenbanken."""
+    cols = {r["name"] for r in conn.execute("PRAGMA table_info(tickets)")}
+    if "archived" not in cols:
+        conn.execute("ALTER TABLE tickets ADD COLUMN archived INTEGER NOT NULL DEFAULT 0")