Tutor MCP v0.4.0 — PostgreSQL backend + stateless multi-node

Tutor MCP v0.4.0

Tutor MCP v0.4.0 adds an opt-in PostgreSQL backend and stateless
multi-node operation, so the runtime can scale horizontally beyond the
single-node SQLite ceiling. SQLite stays the default — existing single-node
deployments are unchanged and require no configuration.

Highlights

• PostgreSQL backend (opt-in). Set DB_DRIVER=postgres + DATABASE_URL to
  run on Postgres via the pure-Go pgx driver (no CGO). The persistence layer was
  inverted behind a store.Store port and a single conformance suite is
  replayed against both backends, so behaviour stays equivalent. Dialect-aware
  schema, ?→$N rebind, RETURNING/ON CONFLICT, and DB_MAX_CONNS pool tuning.
• Stateless multi-node. Run N instances behind a load balancer against a
  shared Postgres:
  • SCHEDULER_MODE=distributed — each scheduled run is leased in the DB, so a
    job/nudge fires exactly once across the fleet.
  • RATELIMIT_BACKEND=postgres — shared rate-limit and login-failure stores
    for coherent fleet-wide throttling and brute-force lockout.
  • JWT_SECRET shared across instances; concurrent cold-start migrations are
    serialized with a Postgres advisory lock.
• Row-level write concurrency on Postgres. The record_interaction hot path
  uses SELECT … FOR UPDATE and the webhook queue uses SKIP LOCKED — the same
  lost-update protection Postgres-side that BEGIN IMMEDIATE gives on SQLite.

Fixes

• First-touch lost update on concept_states under concurrency on Postgres
  (new GetOrCreateConceptStateForUpdate materializes the row before locking).
• nil-pointer panic when a self-transacting method runs inside a WithTx
  callback (new inTx composes onto the current transaction).
• Postgres migration anti-drift guard: MigratePostgres records the schema
  checksum and refuses to boot if schema_pg.sql drifted (SQLite parity).

All three are covered by regression tests, green on real PostgreSQL and SQLite.

Upgrading

• No action required for SQLite users — DB_DRIVER defaults to sqlite and
  behaviour is unchanged.
• To adopt Postgres, see the multi-node runbook in
  OPERATIONS.md and the configuration table in
  README.md.

Operational Notes

• Install URL remains unchanged:

curl -fsSL https://tutor-mcp.dev/install.sh | sh

• latest release assets keep stable names such as
  tutor-mcp_linux_amd64.tar.gz, so installers do not need to know the tag.

Validation

• go build ./...
• go test ./... (SQLite default and against a real PostgreSQL 17 via
  TUTOR_TEST_PG_DSN)

Full changelog: CHANGELOG.md.