feat: persist each player's game so it survives restarts

[dmccoystephenson]

Summary

Per-player game state lived only in memory (SessionService's ConcurrentHashMap), so every backend restart or redeploy wiped everyone's in-progress game — and the docs contradicted themselves about whether games were saved. This persists each player's game to a database, keyed by username.

This is the persistence foundation that unblocks #53 (run history) and #55 (per-account layout).

How it works

• SessionService is now a write-through cache over a new SavedGameRepository: a session is loaded from the DB on a cache miss, and persisted on creation and after every mutation (tick, command, reset, policy change).
• Storage: embedded H2 written to ./data by default (mount as a volume in prod); DB_URL (+ DB_USERNAME/DB_PASSWORD) switches it to Postgres. Chosen over a separate Postgres container to avoid adding memory pressure on the 4 GB prod box (see gateway#105).
• Serialization: the game is stored as JSON. GameState/Army/Tile gain @NoArgsConstructor so Jackson can read them back (they were previously only ever serialized); a lenient mapper tolerates derived getters (getWidth, isMoving). Army.ensureIdsAbove advances the static id counter past restored armies so a post-reload split can't reuse an id.

Modules touched

backend only (+ root docker-compose.yml and docs).

Test plan

• Backend ./mvnw test — 170 passing, incl.:
  • GameStateSerializationTest — a generated game survives a JSON round trip (grid, armies, policies).
  • SessionPersistenceTest — a saved game is reloaded by a fresh SessionService (standing in for a restart).
• Runtime smoke: backend boots with file-based H2, creates ./data/barony.mv.db, serves /state 200.
• Live: play a few turns, redeploy/restart the backend, log back in — the game resumes at the same turn.

Deploy note

The backend image now bundles JPA + H2 + the Postgres driver. The gateway compose needs a data volume mounted at the backend's /app/data so the H2 file persists across container recreation (will add in the gateway PR). backend/pom.xml changed (new deps) — flagging for review.

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