fix: enforce single-view on burn-after-read pastes server-side

Previously the burn was triggered client-side with a 1.5 s delay,
leaving a window where opening the paste in two tabs (or two quick
requests) would both successfully serve the content before either
burn request reached the server.

Fix: on GET /<paste_id> for a burn paste, the server now does an
atomic conditional UPDATE:

    UPDATE pastes SET burn_claimed=1, views=views+1
    WHERE id=? AND burn_claimed=0

SQLite serialises writes, so only one concurrent request can ever
see rowcount > 0. The first request claims the slot and gets the
page; every subsequent request immediately gets 410 Gone — the
encrypted blob is never served to them.

The client-side /api/burn call still fires after successful
decryption to physically delete the row, keeping the data lifetime
as short as possible.

app.py

@@ -136,7 +136,8 @@ def init_db():
                     deletion_token      TEXT,
                     discussions_enabled INTEGER DEFAULT 0,
                     burn_after_read     INTEGER DEFAULT 0,
-                    burn_token          TEXT
+                    burn_token          TEXT,
+                    burn_claimed        INTEGER DEFAULT 0
                 )
             ''')
             # Migration: add deletion_token column to existing table if missing
@@ -145,11 +146,13 @@ def init_db():
             # Migration: add discussions_enabled column to existing table if missing
             if columns and 'id' in columns and 'discussions_enabled' not in columns:
                 cursor.execute('ALTER TABLE pastes ADD COLUMN discussions_enabled INTEGER DEFAULT 0')
-            # Migration: add burn_after_read / burn_token columns to existing table if missing
+            # Migration: add burn_after_read / burn_token / burn_claimed columns to existing table if missing
             if columns and 'id' in columns and 'burn_after_read' not in columns:
                 cursor.execute('ALTER TABLE pastes ADD COLUMN burn_after_read INTEGER DEFAULT 0')
             if columns and 'id' in columns and 'burn_token' not in columns:
                 cursor.execute('ALTER TABLE pastes ADD COLUMN burn_token TEXT')
+            if columns and 'id' in columns and 'burn_claimed' not in columns:
+                cursor.execute('ALTER TABLE pastes ADD COLUMN burn_claimed INTEGER DEFAULT 0')
             cursor.execute('''
                 CREATE TABLE IF NOT EXISTS rate_limits (
                     ip_address    TEXT,
@@ -413,15 +416,36 @@ def view_paste(paste_id):
     if not _PASTE_ID_RE.match(paste_id):
         abort(404)
     paste = _get_paste_or_abort(paste_id)
-    # Increment view count in a separate connection after expiry check.
-    conn = get_db_connection()
-    try:
-        conn.execute('UPDATE pastes SET views = views + 1 WHERE id = ?', (paste_id,))
-        conn.commit()
-    finally:
-        conn.close()
+
     burn_after_read = bool(paste['burn_after_read'])
     burn_token      = paste['burn_token'] if burn_after_read else None
+
+    if burn_after_read:
+        # Atomically claim this burn slot. Only the first request where
+        # burn_claimed=0 succeeds; any concurrent or subsequent request
+        # finds rowcount=0 and gets a 410. SQLite serialises writes so
+        # two simultaneous GETs cannot both claim the slot.
+        conn = get_db_connection()
+        try:
+            res = conn.execute(
+                'UPDATE pastes SET burn_claimed=1, views=views+1 WHERE id=? AND burn_claimed=0',
+                (paste_id,)
+            )
+            conn.commit()
+            claimed = res.rowcount > 0
+        finally:
+            conn.close()
+        if not claimed:
+            abort(410)  # Already viewed — paste is gone
+    else:
+        # Normal paste — just increment view count.
+        conn = get_db_connection()
+        try:
+            conn.execute('UPDATE pastes SET views = views + 1 WHERE id = ?', (paste_id,))
+            conn.commit()
+        finally:
+            conn.close()
+
     return render_template('view.html', paste=paste,
                            burn_after_read=burn_after_read, burn_token=burn_token)