feat: add sqlite account storage and lossless game snapshots

[Enn3Developer]

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[claude]

Claude finished @Enn3Developer's task in 36s —— View job

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Summary

Adds account persistence and lossless game snapshots so the server can survive restarts: a SQLite-backed ServerStore manages accounts/sessions/state, and Game gains snapshot/restore support for saving and reloading in-progress matches.

Changes

• Common: Game.Persistence.cs adds Game.ToSnapshot()/Game.Restore(...) to capture and rebuild a full match (grid, troops, cities, players, tech tree progress) without re-running normal game setup.
• Common: GameSnapshot.cs/PlayerSnapshot define the lossless, versioned (CURRENT_VERSION) DTO for a captured game.
• Common: SnapshotEncoding.cs packs/unpacks the tile and troop arrays to/from little-endian byte blobs for DB storage.
• Common: Score.cs adds an internal Restore(int) to overwrite a score during game restoration.
• Server: ServerStore.cs adds a SQLite-backed store for accounts (PBKDF2-hashed passwords), sessions (hashed tokens, 30-day expiry, timing-safe login), and an opaque server-state blob (SaveState/LoadState).
• Server csproj: adds Microsoft.Data.Sqlite and SQLitePCLRaw.bundle_e_sqlite3 dependencies.
• Tests: GamePersistenceTest.cs and ServerStoreTest.cs cover snapshot round-tripping/validation and account/session/state behavior.

Worth a look

• Game.Restore performs extensive validation (version, grid size, player id range/uniqueness, tribe registration, tech node membership, current-turn player existence) — worth checking all failure paths are actually exercised by the new tests.
• ServerStore schema is versioned via PRAGMA user_version but only has a single migration step (create-from-scratch); there's no upgrade path beyond version 1 yet, which is fine for now but will need attention once the schema changes.
• server_state is stored as a single opaque row (id=0) — no history/versioning of past states, so a corrupt or unexpected write there is unrecoverable other than via backup.