[Rust port] relayburn-ledger: list_content_session_ids + Config / loadConfig surface

[willwashburn]

Parent: #243 / #259

Context

relayburn-ingest (#245) needs two read APIs from relayburn-ledger that
exist in the TS @relayburn/ledger but not yet on the Rust side after the
SQLite redesign:

1. Ledger::list_content_session_ids() -> Result<HashSet<String>>:
   distinct session_id values present in content.sqlite. The TS
   adapter exposes this as listContentSessionIds(). Powers the skip
   filter in reingest_missing_content ([Rust port] relayburn-ingest: gap-warning state machine + reingest_missing_content #278).
2. loadConfig() -> Result<BurnConfig>: loads the user-level config
   that includes content.store: 'off' | 'summary' | 'full'. Powers
   the parser-options decision in every per-harness ingest helper, plus
   the gap-warning gating in [Rust port] relayburn-ingest: gap-warning state machine + reingest_missing_content #278.

Scope

• Add Ledger::list_content_session_ids (or a query_content_session_ids
  free function) that runs SELECT DISTINCT session_id FROM content and
  returns the set.
• Decide the home for the Config type and load_config. Either a new
  relayburn-config crate (TS @relayburn/ledger co-locates it; Rust
  could too) or extend relayburn-ledger. Schema mirrors
  packages/ledger/src/config.ts.

Acceptance

• cargo test -p relayburn-ledger covers both APIs.
• relayburn-ingest ([Rust port] relayburn-ingest: per-harness orchestration loops #277, [Rust port] relayburn-ingest: gap-warning state machine + reingest_missing_content #278) consumes the new APIs without a
  circular dep.