Finalize local message hashes and accept SHA-256 API references - #45

Local-only sends and reactions acquire their protocol SHA-256 atomically with time_sent. Mutations share a transaction and row lock, stage file writes, and return success after commit. The shared finalizer retains the immutable wire representation for federation and independently finalizes add-to batches.

Expose sha256/psha256 in message, list, WebSocket and structured thread responses, preserving message_sha256 as an alias. Message routes accept numeric IDs or SHA-256. Request pid also accepts original or batch hashes while response pid stays numeric. Participation checks apply to the referenced branch; batch-only participants use the batch hash to reply.

Depends on markmnl/fmsgd#44 (module pinned to its feature commit) and implements markmnl/fmsg#37. Existing installations stop both services and run the standalone fmsg-backfill binary, which embeds the schema upgrade and converts existing data atomically. Start the matching services after migration succeeds. The runtime requires finalized storage; only drafts expose null message hashes. Docker integration: markmnl/fmsg-docker#21.

Validation: builds, Go tests, PostgreSQL integration tests, race checks and vet passed. Tests cover concurrent sends/edits, rollback and commit rejection, hash authorization, batch replies and WebSocket payloads. The coordinated Docker suite passed all 15 tests with forced challenges, including later federation of locally delivered compressed messages. CI runs the PostgreSQL tests against the schema from the pinned daemon module.