feat(models): add Claude Fable 5.1 and correct Sonnet 5 pricing

Register `claude-fable-5-1` in the CLIProxy model catalog, the dashboard
catalog, and the usage pricing registry.

Pricing is taken from Anthropic's official pricing page:

- Fable 5.1 base rates are $10/$50 per MTok with a $12.50 5m cache write,
  matching Fable 5.
- Cache hits bill at 0.025x base input ($0.25/MTok) rather than the
  standard 0.1x multiplier. Anthropic applies that reduced rate only to
  Fable 5.1 and Mythos 5.1, so this entry cannot derive its cache rates
  from CACHE_READ_MULTIPLIER.

This also corrects Claude Sonnet 5 from $3/$15 to $2/$10 (cache write
$2.50, cache read $0.20). The launch introductory rate became the
standard price and the increase scheduled for 2026-09-01 was cancelled,
so the previous entry over-reported Sonnet 5 usage cost by 50%.

Thinking on Fable 5.1 is always on and can only be steered through
effort levels, so the catalog entry exposes the same `low`..`max` level
surface as Fable 5 and Opus 5 instead of a manual token budget.

The GitHub Copilot catalog is deliberately left unchanged, since Copilot
availability for Fable 5.1 is not verified; surfacing it there would
offer a model the backend may reject.

Regenerates docs/reports/hardening-inventory.{json,md} so the ci-parity
gate matches the source tree.

docs/reports/hardening-inventory.json

@@ -739,7 +739,7 @@
         },
         {
           "file": "src/web-server/model-pricing.ts",
-          "loc": 1127
+          "loc": 1138
         },
         {
           "file": "src/cliproxy/config/generator.ts",
@@ -778,8 +778,8 @@
           "loc": 939
         },
         {
-          "file": "src/cliproxy/accounts/registry.ts",
-          "loc": 871
+          "file": "src/cliproxy/model-catalog.ts",
+          "loc": 884
         }
       ]
     }

docs/reports/hardening-inventory.md

@@ -93,7 +93,7 @@ Scope: `src/**/*.{ts,tsx,js,jsx,mjs,cjs}`
 | `src/web-server/routes/cliproxy-auth-routes.ts` | 1531 |
 | `src/cliproxy/auth/oauth-handler.ts` | 1510 |
 | `src/cursor/cursor-executor.ts` | 1234 |
-| `src/web-server/model-pricing.ts` | 1127 |
+| `src/web-server/model-pricing.ts` | 1138 |
 | `src/cliproxy/config/generator.ts` | 1109 |
 | `src/cliproxy/auth/oauth-process.ts` | 1048 |
 | `src/cliproxy/config/env-builder.ts` | 1045 |
@@ -103,5 +103,5 @@ Scope: `src/**/*.{ts,tsx,js,jsx,mjs,cjs}`
 | `src/cliproxy/quota/quota-manager.ts` | 954 |
 | `src/web-server/services/codex-dashboard-service.ts` | 940 |
 | `src/glmt/glmt-proxy.ts` | 939 |
-| `src/cliproxy/accounts/registry.ts` | 871 |
+| `src/cliproxy/model-catalog.ts` | 884 |