feat(graphile-function-bindings): expose API-bound functions as GraphQL mutations

[pyramation]

Summary

Track B2 of constructive-io/constructive-planning#1149.

New graphile/graphile-function-bindings package: a PostGraphile v5 plugin that, at gather time, loads function_api_bindings ⋈ function_definitions for the configured apiId and emits one mutation per graphql-enabled binding:

<alias>(input: <Alias>Input!): <Alias>Payload
# Payload: { clientMutationId, invocationId: UUID, status, invocation: FunctionInvocation }

• Per-protocol control via binding config jsonb: absent graphql key ⇒ enabled; "graphql": false ⇒ not exposed.
• Input derivation (src/derive.ts, shared source for future REST validation, no runtime validation):
  1. JSON Schema on config.schema/config.payloadSchema → typed input object (string/number/boolean/enum/array, required ⇒ non-null)
  2. else payload_args ([{name, type}]) → nullable scalar fields
  3. else fallback input { payload: JSON } passed through verbatim
• Resolver = plain parameterized INSERT INTO <schema>.function_invocations (database_id, task_identifier, payload) via the Grafast context's withPgClient/pgSettings — RLS-enforced on the normal authenticated connection; the plugin never sets jwt.claims.* (server layer supplies jwt.claims.api_id provenance).
• invocation field reuses the generated FunctionInvocation type via pgRegistry resource lookup (resource.get({ id })) when the table is exposed.
• Server wiring: graphql/server/src/middleware/graphile.ts buildPreset(..., apiId) adds createFunctionBindingsPlugin({ apiId }) per API handler.
• Schema invalidation: bindings load once per schema build; new/changed bindings appear on the next handler rebuild (existing graphile-cache invalidation or restart). TODO noted for a NOTIFY on binding changes.

Tests (graphile-test + seeded fn_test schema): mutation-per-binding schema snapshot, disabled/other-api bindings omitted, payload_args + JSON Schema input introspection, JSON fallback, and resolver insert returning the created invocation. pnpm test 22/22, package + server builds green.

Link to Devin session: https://app.devin.ai/sessions/0aa9b0eb80e24f57abbe55773acaa4f5
Requested by: @pyramation

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