feat(workspace): full server + sync implementation, all tests green

- solitaire_server: Axum auth, sync push/pull, leaderboard, daily
  challenge, account deletion, JWT middleware, rate limiting via
  tower_governor, SQLite migrations, health endpoint
- solitaire_server: expose build_test_router (no rate limiting) so
  integration tests work without a peer IP in oneshot requests
- solitaire_sync: SyncPayload, merge logic, shared API types
- solitaire_data: SyncProvider trait, LocalOnlyProvider,
  SolitaireServerClient, auth_tokens keyring integration, blanket
  Box<dyn SyncProvider> impl
- solitaire_data/settings: derive Default on SyncBackend (clippy fix)
- .sqlx/: offline query cache so server compiles without a live DB
- sqlx: removed non-existent "offline" feature flag
- keyring v2: fixed Entry::new() returning Result<Entry>
- sqlx 0.8: all SQLite TEXT columns wrapped in Option<T>
- Integration tests: max_connections(1) on in-memory pool so all
  connections share the same schema

All 191 tests pass; cargo clippy -D warnings clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,32 @@
+# Stage 1 — builder
+# Compiles the solitaire_server binary in release mode.
+# Requires a pre-generated .sqlx/ query cache (run `cargo sqlx prepare --workspace`
+# before building the image so sqlx macros work without a live database).
+FROM rust:slim AS builder
+
+WORKDIR /app
+
+RUN apt-get update \
+    && apt-get install -y pkg-config libssl-dev \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY . .
+
+# Tell sqlx to use the cached query metadata instead of a live database.
+ENV SQLX_OFFLINE=true
+
+RUN cargo build --release -p solitaire_server
+
+# Stage 2 — runtime
+# Minimal image that only contains the compiled binary and its runtime deps.
+FROM debian:bookworm-slim
+
+RUN apt-get update \
+    && apt-get install -y libssl3 ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY --from=builder /app/target/release/solitaire_server /usr/local/bin/solitaire_server
+
+EXPOSE 8080
+
+ENTRYPOINT ["/usr/local/bin/solitaire_server"]