fix: share PoW HMAC secret across gunicorn workers

get_pow_secret() generated a random secret per process, so challenges
signed by worker A failed verification on worker B (~90% failure rate
with 2 workers).  Persist a file-backed secret to data/.pow_secret
using O_EXCL for atomic creation.  FLASKPASTE_POW_SECRET env var
still takes priority when configured.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/config.py

@@ -1,12 +1,35 @@
 """Application configuration."""
 
 import os
+import secrets as _secrets
 from pathlib import Path
 
 # Application version
 VERSION = "1.5.2"
 
 
+def _get_pow_secret(data_dir: Path) -> str:
+    """Read or create a shared PoW signing secret.
+
+    Ensures all gunicorn workers use the same HMAC key by persisting
+    the generated secret to a file in the data directory.  Uses O_EXCL
+    for atomic creation so concurrent workers converge on one secret.
+    """
+    secret_file = data_dir / ".pow_secret"
+    try:
+        return secret_file.read_text().strip()
+    except FileNotFoundError:
+        data_dir.mkdir(parents=True, exist_ok=True)
+        secret = _secrets.token_hex(32)
+        try:
+            fd = os.open(str(secret_file), os.O_WRONLY | os.O_CREAT | os.O_EXCL, 0o600)
+            os.write(fd, secret.encode())
+            os.close(fd)
+            return secret
+        except FileExistsError:
+            return secret_file.read_text().strip()
+
+
 class Config:
     """Base configuration."""
 
@@ -70,8 +93,12 @@ class Config:
     # Difficulty is number of leading zero bits required in hash (0 = disabled).
     POW_DIFFICULTY = int(os.environ.get("FLASKPASTE_POW_DIFFICULTY", "20"))
     POW_CHALLENGE_TTL = int(os.environ.get("FLASKPASTE_POW_TTL", "300"))  # 5 minutes
-    # Secret key for signing challenges (auto-generated if not set)
-    POW_SECRET = os.environ.get("FLASKPASTE_POW_SECRET", "")
+    # Secret key for signing challenges.
+    # When not set via env, a shared secret is persisted to data/.pow_secret
+    # so all gunicorn workers use the same HMAC key.
+    POW_SECRET = os.environ.get("FLASKPASTE_POW_SECRET", "") or _get_pow_secret(
+        Path(os.environ.get("FLASKPASTE_DB", Path(__file__).parent.parent / "data" / "pastes.db")).parent
+    )
     # Registration PoW difficulty (higher than paste creation for security)
     REGISTER_POW_DIFFICULTY = int(os.environ.get("FLASKPASTE_REGISTER_POW", "24"))
 
@@ -159,6 +186,7 @@ class TestingConfig(Config):
 
     TESTING = True
     DATABASE = ":memory:"
+    POW_SECRET = "test-pow-secret"
 
     # Relaxed dedup for testing (100 per second window)
     CONTENT_DEDUP_WINDOW = 1