feat(managementapi): repository create handler (S17 Phase 1 Step 9)

Implements Step 9 of the S17 Phase 1 plan: the POST /api/v1/{slug}/repositories create handler, replacing the Step 2 placeholder.

• Strict body decode (DisallowUnknownFields, trailing-data rejection per error case E-2) and full validation before any database access: name rule, format/visibility enums, description length, visibility defaults private, kind defaults hosted.
• 201 with the serialized resource and root-relative Location; 400 bad_request on structural violations; 422 unprocessable_entity on any non-hosted kind (per the contract) and on the ADR-004 per-format cap; 409 conflict on a name collision; detail-free 500 with the cause logged, cancellations classified out (no log, no write), matching the Step 8 read handlers.
• Deps gains the Creator seam next to Step 8's reader; wiring shares one RepositoryStore and keeps the fail-fast nil checks and compile-time interface assertions.
• Every response-asserting test also validates against api/openapi/v1.yaml through the shared contract helpers; a DB-backed composition test drives POST through the production stores (201, then 409 on the same name).

Size

1,188 changed lines: 317 production (create.go 220, request.go 59, handler.go 22, wiring 16), 871 tests. The reviewable production surface is within the guideline; the overage is test code, per the documented test-first authorship flow.

Spec coverage

Spec: docs/specs/S17-rest-management-api.md

Acceptance criteria

#	Criterion	Tests
AC-1	OpenAPI document defines endpoints and validates in CI	Step 1 (merged); CI-owned. Not this MR.
AC-2	POST creates each format: 201, resource body, Location; three-row transaction with full rollback	TestCreateHandler_CreatesEachFormat (handler half); transactional insert and rollback owned by Step 6 (repositories_integration_test.go)
AC-3	POST against an existing name returns 409	TestCreateHandler_NameConflict_Returns409
AC-4	Non-hosted kind 422; invalid name/format/visibility, over-long description, malformed body 400; omitted visibility defaults to private	TestCreateHandler_NonHostedKind_Returns422, TestCreateHandler_Validation_Returns400, TestCreateHandler_ValidationBoundaries_Accepted, TestCreateHandler_Defaults
AC-5	Per-format cap rejects with 422	TestCreateHandler_CapReached_Returns422; the handler-supplied ADR-004 value asserted in TestCreateHandler_CreatesEachFormat; cap counting owned by Step 6
AC-6	format=maven creates; container and npm likewise	TestCreateHandler_CreatesEachFormat
AC-7	GET detail 200/404, no settings key	Sibling Step 8 MR. Not this MR.
AC-8	GET list filters, sorts, 400 on invalid params	Sibling Step 8 MR. Not this MR.
AC-9	GET list keyset pagination	Sibling Step 8 MR. Not this MR.
AC-10	PATCH updates, 422 on immutable field	Sibling Step 10 MR. Not this MR.
AC-11	DELETE hard-deletes, 409 with artifacts, 404 on miss	Sibling Step 10 MR. Not this MR.
AC-12	Cross-slug isolation	Create side: TestCreateHandler_ScopesToResolvedNamespace, TestCreateHandler_UnknownSlug_404AndNoStoreCall; read/update/delete arms owned by Steps 8/10
AC-13	Every error path returns the S01 envelope with request_id	assertErrorEnvelope asserted in every create-handler error test (400, 404, 409, 422, 500)

Error cases

#	Condition	Tests
E-1	Slug resolves to no namespace: 404 not_found	TestCreateHandler_UnknownSlug_404AndNoStoreCall; TestHandler_UnknownSlug_Returns404WithEnvelope (POST row, pre-existing)
E-2	Malformed JSON body: 400	TestCreateHandler_Validation_Returns400 (malformed, empty, non-object rows)
E-3	Missing/invalid authentication: 401 (S08)	Central auth middleware (S08 stub). Not tested in this MR.
E-4	Not permitted: 403 (S09)	Central authz middleware (S09 stub). Not tested in this MR.
E-5	List invalid format/kind/sort/order/limit: 400	Sibling Step 8 MR. Not this MR.
E-6	Create name conflict: 409 conflict	TestCreateHandler_NameConflict_Returns409
E-7	Create invalid name, format, visibility, or over-long description: 400	TestCreateHandler_Validation_Returns400; accepting edges in TestCreateHandler_ValidationBoundaries_Accepted
E-8	Create non-hosted kind: 422	TestCreateHandler_NonHostedKind_Returns422
E-9	Create per-format cap reached: 422	TestCreateHandler_CapReached_Returns422
E-10	Detail/Update/Delete repository missing: 404	Sibling Steps 8/10 MRs. Not this MR.
E-11	Delete with artifacts: 409	Sibling Step 10 MR. Not this MR.
E-12	Update immutable field: 422	Sibling Step 10 MR. Not this MR.
E-13	Update invalid field value: 400	Sibling Step 10 MR. Not this MR.
E-14	Unexpected server failure: 500	TestCreateHandler_StoreFailure_Returns500, TestCreateHandler_StoreFailure_LogsCause, TestCreateHandler_StoreContractViolations_Return500

Security considerations

#	Concern	Tests
S-1	Auth (S08) and authz (S09) on every endpoint	Central middleware stubs, wired outside this handler. Not tested in this MR.
S-2	Existence hiding: unresolved slug is 404	TestCreateHandler_UnknownSlug_404AndNoStoreCall
S-3	Input validation before any database access; URL-path-safe names	TestCreateHandler_Validation_Returns400 and TestCreateHandler_NonHostedKind_Returns422 (empty creator.inputs() after rejection); path-smuggling row name with path separator
S-4	Injection: Jet builder with bound parameters	Datastore-owned (Step 6). Not tested at this seam.
S-5	Name immutability	Update-side concern (sibling Step 10). Create-side name rules covered under S-3.
S-6	Tenant isolation: queries scoped by resolved namespace_id	TestCreateHandler_ScopesToResolvedNamespace; query-side scoping owned by Step 6 integration tests

Related to #172 (closed)