refactor: split 4 files exceeding 800-line limit

[forkwright]

Summary

• harmonia-db play_history.rs (1575 lines) → play_history/{mod,tests}.rs; tests extracted as #[cfg(test)] mod tests; submodule
• paroche opds/catalog.rs (1161 lines) → opds/catalog/{mod,tests}.rs; same pattern
• harmonia-host tests/acquisition_integration.rs (1116 lines) → pipeline and startup-recovery tests extracted into tests/acquisition_integration/{pipeline_tests,recovery_tests}.rs via #[path] attributes
• komide service.rs (817 lines) → service/{mod,tests}.rs; same pattern as the db/paroche splits

Test plan

• cargo fmt --all -- --check passes
• cargo clippy --workspace --all-targets -- -D warnings passes
• cargo test --workspace passes (all tests pass, verified locally)
• cargo test --workspace --doc passes
• kanon lint . --rust reports zero RUST/file-too-long violations

Closes #1613

Observations

• The task description mentions 45 files exceeding 800 lines; kanon lint found exactly 4 at the time of this work. The remaining 41 may be tracked under a different lint rule or were fixed separately.
• acquisition_integration.rs required #[path = "..."] attributes because Rust 2024 module path resolution for integration test root files looks in the same directory, not a subdirectory named after the file. The #[path] attributes are explicit and correct.
• A note in pipeline_retry_budget_exhaustion_marks_failed documents a known bug in SyntaxisService where ActiveEntry.retry_count resets to 0 on re-load; this was pre-existing and is preserved as-is.

[github-actions]

⚠️ Large PR detected — 10 files, 5285 lines changed.

Consider splitting into smaller PRs for easier review. Not a blocker, just a signal.