feat: recover match salts via Steam Game Coordinator

[raimannma]

What

Adds a src/gc/ module that logs into remembered Steam accounts through the user's own session and recovers match salts via the GC GetMatchMetaData message — independently of the existing httpcache scraper — POSTing them to the same /v1/matches/salts endpoint (tagged ingest-tool:{steam_id3}). Ported from the mod-manager feat/match-data-sharing branch.

How it works

• Source: pulls the global /v1/matches/to-fetch list and picks up to the remaining quota (newest-first, deduped across accounts within a pass). Not the player's own matches.
• Rate limit: 40 fetches per account per 24h, enforced by a persisted rolling-window quota (~/.local/share/deadlock-api-ingest/gc-quota.json) so the cap survives restarts. Requests are throttled 2 min apart (Steam rate-limits the GC per account).
• Multi-account: iterates every account whose refresh token can be decrypted from local.vdf (AES on Linux/macOS, DPAPI on Windows), each with its own quota bucket.
• Accounts that can't do requests: undecryptable accounts are skipped at recovery; an account whose GC handshake fails (e.g. doesn't own Deadlock) is backed off for 24h and the batch moves on; Steam rate-limiting exhausts only that account's window.
• Safety: never runs while Deadlock is running (Steam routes GC traffic to the game's own pipe), re-checked mid-pass. Refresh tokens are in-memory only with a redacted Debug impl — never logged, persisted, or sent.

Wiring

• Runs one pass in --once mode, a 30-min background worker in watch mode, and is skipped in game-wrapper mode (game is running).
• New --no-gc flag mirrors --no-statlocker.

Module layout

auth (session recovery) · client (async steam-vent GC session) · api (to-fetch + post, blocking ureq) · quota (pure rolling window, unit-tested) · store (persisted quota + backoff) · mod (orchestration).

Notes

• The 40/24h cap is per-account (matching the source and Valve's per-account GC rate limit). With N usable accounts that's up to N×40/day — easy to switch to a single global budget if preferred.
• Adds git deps steam-vent and valveprotos (same revs as the mod-manager branch). Cargo.lock regenerated from master's baseline; the Nix build is naersk/Cargo.lock-driven, so no cargoHash bump needed.

Builds clean, passes clippy under the crate's pedantic/correctness denies, and the test suite is green.

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