Add ingester staging-directory config and path safety validation

[ian-ross]

Part of #3.

Implement the configuration and path-safety foundation for ingester database isolation.

Decisions

• Add a required paths.staging-directory option.
• Preserve existing semantics:
  • data-directory: shared/public SQLite snapshots for clients.
  • staging-directory: durable local ingester-owned SQLite files; must survive ingester restart.
  • scratch-directory: ephemeral receiver/export scratch; may be deleted between restarts.
• All three path roots must be distinct and none may be nested inside another, using resolved/real paths.
• Config should validate path configuration but should not create directories.
• DBCache should also validate paths because it can be instantiated directly.
• DBCache should require data-directory to exist and may create staging-directory and dedicated export scratch subdir as needed.

Implementation checklist

• Add staging_directory to Config._init_paths() from paths.staging-directory.
• Add required path distinct/non-nested validation in Config.
• Add duplicate validation in DBCache.__init__().
• Update config.toml.template comments to document all three path roles.
• Update feder-ingest config wiring to pass data_directory, staging_directory, and scratch_directory explicitly to DBCache.
• Add ingester config values:
  • ingester_export_interval, default 1 hour.
  • ingester_finalize_after, default 12 hours.
• Pass those values explicitly to DBCache.

Tests

• Config requires paths.staging-directory.
• Config rejects equal or nested combinations of data-directory, staging-directory, and scratch-directory.
• DBCache also rejects equal/nested path roots when constructed directly.

Notes

This is the safety boundary for #3: public data, durable staging, and ephemeral scratch must be separate so the ingester cannot accidentally expose live SQLite/WAL files to clients.

[ian-ross]

Implemented in 2bcc44c: added required staging-directory config, ingester lifecycle defaults, path safety validation in Config and DBCache, and tests.