feat: caller dictates the cache log path; drop RUN_DIR

[swelborn]

The orchestrator now owns where each cache writes its log, so the cache
runs in the per-transfer work dir alongside the producer's output.

• CacheRequest gains a required absolute log_path.
• start_cache(cache_id, config, log_path) writes config.json into
  log_path.parent and launches the binary with cwd set to that dir.
• remove the RUN_DIR setting (and .env.example entry); keep FASTCACHE_BINARY.
• create_cache route forwards req.log_path.

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This is part 3 of 10 in a stack made with GitButler:

•  10  remove jwt/jwks from this #14
•  9  add idle timeout option #13
•  8  refactor into functional core/imperative shell #12
•  7  add alembic for migrations #11
•  6  use a key to dedupe reqs #10
•  5  use anyio and fixup process handling #9
•  4  systemd + vault certs #8
•  3  feat: caller dictates the cache log path; drop RUN_DIR #7 👈
•  2  minor model updates and configuration change since timeout was added #6
•  1  create ports locally to fastcache server #5