feat: add json output exposing the GeneratorMetadata contract

[spydon]

What kind of change does this PR introduce?

Feature, stacked on #1084 (base branch is its claude/funny-tesla-wofjdj head; only the last commit is new). Adds a json output alongside typescript, go, swift, and python, emitting the raw GeneratorMetadata contract of @supabase/postgrest-typegen that #1084 makes postgres-meta consume.

What is the new behavior?

• GET /generators/json returns the introspected GeneratorMetadata (schemas, tables, foreign tables, views, materialized views, columns, relationships, functions, types) as pretty-printed JSON, with the same included_schemas/excluded_schemas query parameters as the other generators.
• PG_META_GENERATE_TYPES=json (and the gen:types:json npm script) emits the same document on stdout, so the Supabase CLI can expose it as supabase gen types --lang json and pipe it into out-of-process generators as --lang dart (feat(gen): add dart and json output languages to gen types cli#6230, feat: add supabase_typegen package generating typed table definitions supabase-flutter#1635 whose parser consumes exactly this contract).

This gives third-party and community type generators a single high-fidelity, language-neutral introspection source: exact nullability, database defaults, identity and generated columns, enum values, views, and materialized views, which lossy sources like the PostgREST OpenAPI description cannot provide.

Additional context

• The output is a plain JSON.stringify over the sortGeneratorMetadata-canonicalized introspection result, so it adds no new queries and stays in sync with whatever the built-in generators see.
• Supersedes feat: add json generator exposing the raw generator metadata #1106 (closed as a duplicate of the feat: consume @supabase/postgrest-typegen for type generation #1084 extraction): same endpoint surface, now backed by the package instead of the deleted in-repo metadata assembly. Unlike feat: add json generator exposing the raw generator metadata #1106 this emits the bare contract without a version envelope; whether the serialized contract should carry one is raised on the postgrest-typegen RFC.
• Tests assert the envelope keys and the fidelity data that motivate the endpoint (nullability, defaults, identity, enum values, views, materialized views), plus schema filtering. The full suite passes locally with the same pre-existing environment-specific failures as master (SSL and query-timeout tests).