feat(relay): implement POST /v1/dids — did:plc genesis op and account promotion (MM-89) · pull #26

Summary#

Adds build_did_plc_genesis_op pure function to the crypto crate — ECDSA-SHA256 signed did:plc genesis operations with RFC 6979 determinism and base32-lowercase DID derivation
Adds POST /v1/dids route to the relay — authenticates via pending session Bearer token, calls plc.directory, then atomically promotes the pending account to a full account
V008 migration: nullable accounts.password_hash (mobile accounts have no password), pending_did column on pending_accounts for pre-store retry resilience

cargo test --workspace — 253 tests, 0 failures (7 crypto unit tests + 7 relay integration tests with wiremock)
cargo clippy --workspace -- -D warnings — zero warnings
cargo fmt --all --check — clean
Human test plan: docs/test-plans/2026-03-13-MM-89.md