feat(ranked): make tier thresholds durable instead of compiled in

Task 8.22. Tier bands lived in domain.DefaultTierPolicy(), compiled into
every API binary, so retuning one meant building and rolling a new image
-- least attractive exactly when it is most needed, as the rating
distribution settles after launch.

Bands now live in a tier_bands table, seeded by the migration with the
exact policy the binaries hardcode, so this changes durable state without
changing behaviour. Retuning is a rolling restart rather than a rebuild.

Three properties the loader deliberately holds:

- A malformed durable policy stops startup. Falling back on error would
  silently mis-tier every player, which is worse than not starting.
- An empty table is supported and falls back to the compiled default, so
  an operator can truncate back to known-good without a deploy, and a
  fresh database works before the seed is reviewed.
- PROVISIONAL is rejected as a band. It is derived from ranked game
  count, not rating, so a band claiming it would be unreachable at best
  and would shadow a real tier at worst.

Bands stay backend-owned; clients still receive only the resulting label,
per docs/MATCHMAKING.md. UNIQUE(min_rating) rejects two bands sharing a
threshold, catching an ambiguous policy before NewTierPolicy does.

testkit-api loads it too, so the control-plane integration scripts
exercise the durable path rather than the compiled default.

Integration tests cover the seeded policy matching the compiled one,
retuning taking effect from the database alone, truncation falling back,
and each invalid-policy shape being rejected. Verified they fail against
a loader that ignores durable bands.

The other two parts of 8.22 needed no work: the client UI already renders
tier, provisional status, ranked games and the season countdown, and
reconnect transport is 8.42's, dependent on live backend events.

multiplayer-next.md

@@ -217,7 +217,7 @@ are done; everything below is what's left on the tasks still open.
 | 8.19 `[D:8.18]` | Casual lineup (2–6 humans, bot backfill) | Queue candidate selection, opt-in 10 s backfill proposals, reconnect/leave penalties, live integration remain |
 | 8.20 `[D:8.18]` | Ranked admission (six unique verified humans) | Done. Allocation wiring was already complete end to end (allocator sets the `cosmic-clash.io/arena-path` annotation → `supervisor.withAllocatedCompatibility` maps it to `--arena-path` → `server_boot.gd` → `ServerMatchLoop.allocated_arena_path`), with coverage at each hop. `ArenaRegistry` integration is now a cross-language guard rather than a shared list: `server/domain/ranked.go` must keep its own ranked-eligible subset (the choice is server-authoritative and made before any Godot process exists), so `arena_registry_sync_test.go` parses `arena_registry.gd` and fails if the two disagree in either direction, if rotation order diverges, or if a ranked path has no scene behind it. Verified against four drift scenarios including promoting an elevated variant, which the registry's own comment anticipates. Live ranked admission against a real cluster remains ([#17](https://github.com/jcreek/CosmicClash/issues/17)) |
 | 8.21 `[D:8.5,8.20]` | Rating core (Glicko-2, weights, transactional updates) | Live maintenance/DB execution remains |
-| 8.22 `[D:8.21]` | Ranked profile (provisional games, tiers) | Persisted tier policy, client UI, reconnect transport remain |
+| 8.22 `[D:8.21]` | Ranked profile (provisional games, tiers) | Persisted tier policy done: bands live in `tier_bands`, seeded with the exact compiled launch policy so storage changed without behaviour changing, loaded at startup with a malformed policy failing startup rather than silently mis-tiering, and an empty table falling back to the compiled default so an operator can truncate back to known-good. Retuning is now a rolling restart rather than a rebuilt image. `PROVISIONAL` is rejected as a durable band, being derived from game count rather than rating. Client UI was already built (`RankedProfileState.display_text()` renders tier, provisional status, ranked games and the season countdown). Reconnect transport is tracked by 8.42 and depends on live auth/backend events |
 | 8.23 `[D:8.21]` | Ranked season policy (compression, rollover) | Live maintenance/DB execution remains |
 | 8.24 `[D:8.9,8.20,8.21]` | Ranked connection policy, reconnect lease, abandon ladder | Live PostgreSQL execution now verified (`make verify-phase6` and every integration script run clean). Process-restart and outage execution remain |
 | 8.25 `[D:8.10,8.24]` | Result policy (workload-bound, idempotent, transactional) | Production credentials, Agones annotation persistence/reconciliation, integrity-evidence adapters remain |