[Bug]: DPoP replay markers grow without bound

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Start a server environment that accepts DPoP-authenticated requests.
2. Send valid DPoP requests with unique jti values.
3. Inspect the configured secrets directory after each request.
4. Separately, submit a token exchange with a valid DPoP proof and an invalid bootstrap credential.

Expected behavior

Replay state should remain atomic while being retained only for the proof acceptance window. A token exchange should validate its bootstrap credential before writing replay state.

Actual behavior

Each accepted proof creates a permanent dpop-proof-<digest>.bin file in the general secrets directory. Normal authenticated traffic adds one file per request, and no cleanup removes them. The token exchange path also records a valid proof before rejecting an invalid bootstrap credential, allowing file creation and fsync work without a usable credential.

Impact

Major degradation over long-running or high-traffic server lifetimes due to unbounded file and inode growth. The finding affects availability and reliability; no credential disclosure was identified.

Version or commit

main at 7963ac740

Investigation

Daybreak Blue confirmed the persistent allocation path and found that replay markers could also be lost across a partial bucket-boundary claim if writes were ordered incorrectly. Fable independently confirmed that normal DPoP traffic reproduces the unbounded growth and recommended a focused reliability fix with bounded, dedicated replay storage and credential preflight.

Proposed fix: #8845

Start in apps/server and reproduce accepted DPoP requests while inspecting the configured secrets directory. Trace replay-marker writes and the token-exchange credential check, then verify that replay state is bounded and atomic, retained only for the acceptance window, and not written before invalid bootstrap credentials are rejected.