fix: add fastify as a peer dependency

[adriano-tirloni]

Summary

• Adds fastify as a peerDependency (>=5.0.0) to fix type resolution issues in strict package managers like pnpm
• @fastify/sensible imports types from fastify in its type declarations (types/index.d.ts) but only lists it in devDependencies
• In pnpm monorepos with multiple fastify versions, this causes incompatible FastifyInstance types and TypeScript errors

Problem

In pnpm (which uses strict isolated node_modules), @fastify/sensible cannot resolve fastify from its own node_modules since it's not a declared dependency. Node's resolution algorithm walks up the directory tree and can land on a different version of fastify installed by another workspace package, causing errors like:

No overload matches this call.
  Type 'FastifyInstance<..., FastifyTypeProvider>' is missing the following properties
  from type 'FastifyInstance<..., FastifyTypeProviderDefault>': ...

Steps to reproduce

1. Create a pnpm monorepo with two apps depending on different fastify versions (e.g., 5.3.3 and 5.8.4)
2. Install @fastify/sensible in one of them
3. Register it with app.register(fastifySensible) in a TypeScript file with skipLibCheck: false
4. TypeScript fails with the overload error above

Fix

This is consistent with other @fastify/* plugins like @fastify/cors, @fastify/cookie, @fastify/helmet, etc. that already declare fastify as a peer dependency.

Test plan

• Existing tests pass (npm test)
• Verify in a pnpm monorepo that the type resolution error is resolved