feat: add sqlite account storage and lossless game snapshots

[Enn3Developer]

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[claude]

Claude finished @Enn3Developer's task in 37s —— View job

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Summary

Adds persistence for the server: a SQLite-backed ServerStore for accounts/sessions/state, and a lossless snapshot format so an in-progress Game can be saved and restored across server restarts.

Changes

• common: Game.ToSnapshot() / Game.Restore() capture and rebuild all mutable game state (grid tiles, troops, cities, players, turn/score/researched techs) while re-deriving static content (tribes, tech tree shape, definitions) from the caller.
• common: GameSnapshot/PlayerSnapshot DTOs plus SnapshotEncoding for little-endian packing of tile/troop/city arrays into BLOBs, versioned via GameSnapshot.CURRENT_VERSION.
• common: Score.Restore(int) added to let Game.Restore set a score directly instead of only accumulating via AddScore.
• server: New ServerStore (SQLite via Microsoft.Data.Sqlite) handling account registration/login/session resume/logout (PBKDF2 password hashing, SHA-256 session tokens, timing-safe compare) and an opaque SaveState/LoadState blob for server state.
• server: OpenPolytopia.Server.csproj adds Microsoft.Data.Sqlite and SQLitePCLRaw.bundle_e_sqlite3 package references.
• tests: New GamePersistenceTest.cs and ServerStoreTest.cs covering snapshot round-tripping, restore validation errors, and store account/session behavior.

Worth a look

• Game.Restore throws on many malformed-snapshot cases (version mismatch, bad grid size, unknown tribe/tech, dangling turn holder) — worth confirming all validation paths are exercised by the new tests.
• ServerStore schema is versioned (SCHEMA_VERSION = 1) but Migrate() only creates the initial schema; there's no upgrade path yet for future schema changes.
• SaveState/LoadState treat the server state as an opaque string blob with no versioning of its own, unlike the game snapshot format.