[pbhandari]

Closes EAP-831.

Replaces the two storage_routing boolean flags with per-policy load thresholds that gate dynamic pardoning.

Changes (one commit each)

1. Always fetch cluster load info — drop storage_routing.enable_get_cluster_loadinfo (option + schema key + the gates in get_cluster_loadinfo and db_query); load info is now fetched unconditionally so dynamism is always configurable.
2. Configurable per-policy thresholds — add a block-level thresholds key to the allocation_policies config (sibling of policies/match/remove). The resolver copies the block's thresholds onto every policy it constructs, so each policy owns its reject/throttle ceilings at runtime. Keys are enumerated in sentry-options/schemas/snuba/schema.json, and a test asserts those keys are a subset of LoadInfo's dataclass fields.
3. Dynamic enforcement — drop storage_routing.enable_dynamic_allocation_policy. LoadInfo.should_pardon() is replaced by LoadInfo.exceeds(thresholds), and the pardon path in QuotaAllowance.decision now pardons a rejection only when the policy is pardonable, load info exists, reject thresholds are configured, and the cluster is below all of them. No thresholds / missing load info means keep rejecting.

Config shape:

allocation_policies:
  EAP:
    - match: {...}
      policies: [...]
      thresholds:
        reject:   { cluster_load: 90, concurrent_queries: 100 }
        throttle: { ... }   # helper built now, consumed later

is_enforced / is_pardonable

• is_enforced=False -> dynamism skipped; policy always passes through (existing short-circuit).
• is_enforced=True + no thresholds -> always enforced (never pardoned).
• is_pardonable is unchanged — still the per-policy toggle for participating in dynamic pardoning.

Out of scope

• The generic-exception path in _get_cluster_loadinfo returns LoadInfo() with all -1s, which compares below any threshold and would wrongly pardon. Fail-safe handling of that case is deferred.
• Wiring thresholds.throttle into throttling decisions (the helper supports it; consumption comes later).

Tests

• tests/query/allocation_policies/test_thresholds_schema.py (new): schema threshold keys subset of LoadInfo fields.
• LoadInfo.exceeds unit tests.
• Updated existing pardon tests (test_allocation_policy_base, test_db_query, test_storage_routing, test_cluster_loadinfo) to the thresholds API.

[linear-code]

EAP-831

[pbhandari]

I don't need all these signals. But this is fine as a temp measure while we figure out what signals to monitor.