ci(web-e2e): prebuild server so Playwright webServer stops timing out

The web-e2e job has been failing on every run with:

    Error: Timed out waiting 120000ms from config.webServer.

Playwright's `webServer` runs `cargo run -p solitaire_server --quiet` and
waits 120s for /health. The workflow cached only npm — no Rust cache — so each
run cold-compiled the entire server graph (axum/sqlx/reqwest/aws-lc-sys) inside
that 120s window and never came up. The browser tests never executed; the
failure was infra, not a web regression.

Fix the harness so the tests actually run:
- add `Swatinem/rust-cache@v2` to warm the cargo cache across runs
- add a `Prebuild server` step (`SQLX_OFFLINE=true cargo build -p
  solitaire_server`) before Playwright, so `webServer`'s `cargo run` reuses the
  compiled binary and starts in seconds (.sqlx offline cache is committed)
- raise `webServer.timeout` 120s -> 300s as a safety margin for a cold cargo
  cache (e.g. first run after a deps bump)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/web-e2e.yml

@@ -25,6 +25,19 @@ jobs:
       - name: Install Rust
         uses: dtolnay/rust-toolchain@stable
 
+      - name: Cache cargo build
+        uses: Swatinem/rust-cache@v2
+
+      # Prebuild the server so Playwright's `webServer` (which runs
+      # `cargo run -p solitaire_server`) starts from a compiled binary instead
+      # of cold-compiling the whole dependency graph (axum/sqlx/reqwest) inside
+      # its 120s startup window — the timeout that was failing every run.
+      # SQLX_OFFLINE uses the checked-in `.sqlx/` query cache (no live DB).
+      - name: Prebuild server
+        env:
+          SQLX_OFFLINE: 'true'
+        run: cargo build -p solitaire_server --quiet
+
       - name: Set up Node.js
         uses: actions/setup-node@v4
         with: