Make player token reset non-breaking (deferred rotation)

The maintenance-page reset used to rotate auth_code immediately. Because the
token also guards the playlist endpoints, that stopped a player from syncing
content the instant the button was pressed -- before the player ever had a
chance to react -- and clients that do not understand the heartbeat's
401 auth_required would have stayed broken until redeployed.

Split the model behaviour:

* request_reauth()  -- sets the flag only (used by the admin reset endpoints
  and the automatic interval). The current token stays valid, so there is no
  outage while the player is being asked to re-authenticate.
* issue_new_token() -- replaces the token now (used by the existing
  "regenerate auth code" action, where the admin is shown the new value).
* mark_authenticated() -- if a reset was pending, this is where the old token
  is retired: a new token is issued and returned to the player in the
  /api/auth/player response.

Net effect: the reset is a request the player acts on, never a forced logout.
A player that ignores it keeps working and shows as "awaiting re-auth" in the
UI; one that handles it gets a fresh token and the old one stops working.

Maintenance page wording updated to match (request vs reset; "awaiting
re-auth" badge), plus a themed info note explaining it is not a forced logout.

Adds docs/player-auth-reset.md describing the protocol and exactly what the
player client (Kiwy-Signage) still needs to implement: detect 401 /
auth_required on the heartbeat (and on the token-guarded endpoints), re-run
POST /api/auth/player with the stored quick-connect code, save the new
auth_code and retry. No password or SSH redeploy required.

Verified end-to-end: token unchanged and still accepted after the request
(200), heartbeat answers 401 auth_required=token_reset, re-auth issues a
different token, the old token is then rejected (403) and the new one accepted
(200), flag cleared; the interval path only flags without rotating.

app/blueprints/api.py

@@ -115,8 +115,12 @@ def player_needs_reauth(player: Player):
     * an explicit reset flag (``auth_reset_required``) set from the admin
       maintenance page, individually or for every player; and
     * the optional automatic expiry interval configured on that page — a token
-      older than ``reset_interval_days`` is rotated and the player is asked to
-      authenticate again.
+      older than ``reset_interval_days`` asks the player to authenticate again.
+
+    Both are *requests*, not enforcement: the flag is raised here and the token
+    is only replaced once the player actually re-authenticates (see
+    ``Player.mark_authenticated``). That keeps a player which cannot handle the
+    request working instead of silently losing playlist sync.
 
     Args:
         player: The player sending the heartbeat.
@@ -139,9 +143,8 @@ def player_needs_reauth(player: Player):
         return False, None
 
     if datetime.utcnow() - issued >= timedelta(days=interval_days):
-        # Expire the token now: rotate it and flag the player, so the response
-        # below tells the client to restart authentication.
-        player.rotate_auth_token(force_reauth=True)
+        # Flag the player; the replacement token is issued when it re-auths.
+        player.request_reauth()
         db.session.commit()
         return True, 'token_expired'
 
@@ -199,10 +202,12 @@ def authenticate_player():
         log_action('warning', f'Failed authentication attempt for hostname: {hostname}')
         return jsonify({'error': 'Invalid credentials'}), 401
     
-    # Update player status and stamp the token. Marking the player as
-    # authenticated clears any pending re-authentication request set from the
-    # admin maintenance page, so a player that just re-authenticated stops
-    # being told to authenticate again.
+    # Update player status and finalise the token. If a re-authentication had
+    # been requested (admin reset or expiry interval), this is where the old
+    # token is retired: mark_authenticated() issues a brand-new one. The value
+    # is returned below as `auth_code`, so the player picks it up here. A
+    # player that re-authenticated with no request pending simply gets its
+    # issue time refreshed.
     player.mark_authenticated()
     player.update_status('online')
     db.session.commit()
@@ -554,11 +559,15 @@ def receive_player_feedback():
             return jsonify({'error': 'Invalid quickconnect code'}), 403
         
         # ------------------------------------------------------------------
-        # Authentication-token reset.
+        # Re-authentication request.
+        #
+        # When an admin requests a token reset (or an automatic reset interval
+        # elapses), the heartbeat is answered with 401 + auth_required so the
+        # client restarts its authentication flow. The player's existing token
+        # is still valid at this point — it is replaced only once the client
+        # actually re-authenticates — so a player that does not understand this
+        # response keeps syncing instead of breaking.
         #
-        # When the admin maintenance page resets a player's token (or an
-        # automatic reset interval has elapsed), the heartbeat is answered with
-        # 401 + auth_required so the client restarts its authentication flow.
         # This runs before any feedback is recorded: the heartbeat is not a
         # meaningful status update while the client is being asked to
         # re-authenticate.
@@ -571,7 +580,7 @@ def receive_player_feedback():
                 'success': False,
                 'auth_required': True,
                 'reason': reauth_reason,
-                'message': 'Authentication token reset. Please re-authenticate.',
+                'message': 'Re-authentication required. Please authenticate again.',
             }), 401
         
         # Create feedback record