Saved games on user profiles

[byte-the-bot]

Implements task BS-219e7007: users can save games to their public profile with an optional title, matching the saved-games feature from play.battlesnake.com.

(Originally stacked on #121 / public-profiles; that PR has since merged, so this now targets main directly and includes only the saved-games work.)

What's included

• Migration: saved_games table — UNIQUE (user_id, game_id) so a user can save a game once; FK'd to users and games. Up + down migrations.
• Model (server/src/models/saved_game.rs): upsert-on-save (re-saving updates the title in place), owner-scoped delete, and a profile listing query joined with games for fallback-title metadata.
• Routes (auth required):
  • POST /games/{id}/save — optional title form field, trimmed and capped at 100 chars server-side; flashes "Game saved to your profile" and redirects back to the game page. 404s for nonexistent games.
  • POST /saved-games/{id}/delete — owner-only (404 for anyone else), flashes and redirects to the owner's /users/{login} profile.
• Game page: logged-in users get a small Save Game form in the aside next to the game details; if they already saved the game it pre-fills their title and the button reads "Update". Diff kept localized since other in-flight PRs touch view.rs.
• Public profile: new "Saved Games" section after Battlesnakes — each entry links to the game, shows the title (falling back to "{game_type} on {board_size}" when untitled) plus the game date, with a Remove button visible only to the profile owner. The section itself is public.

Testing

• #[sqlx::test] model tests: upsert behavior (same row updated on re-save, separate rows per user) and owner-scoped delete.
• Unit tests for title normalization (trim, 100-char cap incl. multi-byte) and the display-title fallback.
• cargo clippy --all-targets warning-free, cargo fmt clean, full suite cargo test -p arena --bin arena green (384 passed).
• .sqlx offline cache regenerated and committed.

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