朱子纯 / vibe-erp

feat(metadata): P3.4 — custom field application (Tier 1 customization)

The keystone of the framework's "key user adds fields without code"
promise. A YAML declaration is now enough to add a typed custom field
to an existing entity, validate it on every save, and surface it to
the SPA / OpenAPI / AI agent. This is the bit that makes vibe_erp a
*framework* instead of a fork-per-customer app.

What landed
-----------
* Extended `MetadataYamlFile` with a top-level `customFields:` section
  and `CustomFieldYaml` + `CustomFieldTypeYaml` wire-format DTOs. The
  YAML uses a flat `kind: decimal / scale: 2` discriminator instead of
  Jackson polymorphic deserialization so plug-in authors don't have to
  nest type configs and api.v1's `FieldType` stays free of Jackson
  imports.
* `MetadataLoader.doLoad` now upserts `metadata__custom_field` rows
  alongside entities/permissions/menus, with the same delete-by-source
  idempotency. `LoadResult` carries the count for the boot log.
* `CustomFieldRegistry` reads every `metadata__custom_field` row from
  the database and builds an in-memory `Map<entityName, List<CustomField>>`
  for the validator's hot path. `refresh()` is called by
  `VibeErpPluginManager` after the initial core load AND after every
  plug-in load, so a freshly-installed plug-in's custom fields are
  immediately enforceable. ConcurrentHashMap under the hood; reads
  are lock-free.
* `ExtJsonValidator` is the on-save half: takes (entityName, ext map),
  walks the declared fields, coerces each value to its native type,
  and returns the canonicalised map (or throws IllegalArgumentException
  with ALL violations joined for a single 400). Per-FieldType rules:
  - String: maxLength enforced.
  - Integer: accepts Number and numeric String, rejects garbage.
  - Decimal: precision/scale enforced; preserved as plain string in
    canonical form so JSON encoding doesn't lose trailing zeros.
  - Boolean: accepts true/false (case-insensitive).
  - Date / DateTime: ISO-8601 parse via java.time.
  - Uuid: java.util.UUID parse.
  - Enum: must be in declared `allowedValues`.
  - Money / Quantity / Json / Reference: pass-through (Reference target
    existence check pending the cross-PBC EntityRegistry seam).
  Unknown ext keys are rejected with the entity's name and the keys
  themselves listed. ALL violations are returned in one response, not
  failing on the first, so a form submitter fixes everything in one
  round-trip.
* `Partner` is the first PBC entity to wire ext through the validator:
  `CreatePartnerRequest` and `UpdatePartnerRequest` accept an
  `ext: Map<String, Any?>?`; `PartnerService.create/update` calls
  `extValidator.validate("Partner", ext)` and persists the canonical
  JSON to the existing `partners__partner.ext` JSONB column;
  `PartnerResponse` parses it back so callers see what they wrote.
* `partners.yml` now declares two custom fields on Partner —
  `partners_credit_limit` (Decimal precision=14, scale=2) and
  `partners_industry` (Enum of printing/publishing/packaging/other) —
  with English and Chinese labels. Tagged `source=core` so the
  framework has something to demo from a fresh boot.
* New public `GET /api/v1/_meta/metadata/custom-fields/{entityName}`
  endpoint serves the api.v1 runtime view of declarations from the
  in-memory registry (so it reflects every refresh) for the SPA's form
  builder, the OpenAPI generator, and the AI agent function catalog.
  The existing `GET /api/v1/_meta/metadata` endpoint also gained a
  `customFields` list.

End-to-end smoke test
---------------------
Reset Postgres, booted the app, verified:
* Boot log: `CustomFieldRegistry: refreshed 2 custom fields across 1
  entities (0 malformed rows skipped)` — twice (after core load and
  after plug-in load).
* `GET /api/v1/_meta/metadata/custom-fields/Partner` → both declarations
  with their labels.
* `POST /api/v1/partners/partners` with `ext = {credit_limit: "50000.00",
  industry: "printing"}` → 201; the response echoes the canonical map.
* `POST` with `ext = {credit_limit: "1.234", industry: "unicycles",
  rogue: "x"}` → 400 with all THREE violations in one body:
  "ext contains undeclared key(s) for 'Partner': [rogue]; ext.partners_credit_limit:
  decimal scale 3 exceeds declared scale 2; ext.partners_industry:
  value 'unicycles' is not in allowed set [printing, publishing,
  packaging, other]".
* `SELECT ext FROM partners__partner WHERE code = 'CUST-EXT-OK'` →
  `{"partners_industry": "printing", "partners_credit_limit": "50000.00"}`
  — the canonical JSON is in the JSONB column verbatim.
* Regression: catalog uoms, identity users, partners list, and the
  printing-shop plug-in's POST /plates (with i18n via Accept-Language:
  zh-CN) all still HTTP 2xx.

Build
-----
* `./gradlew build`: 13 subprojects, 129 unit tests (was 118), all
  green. The 11 new tests cover each FieldType variant, multi-violation
  reporting, required missing rejection, unknown key rejection, and the
  null-value-dropped case.

What was deferred
-----------------
* JPA listener auto-validation. Right now PartnerService explicitly
  calls extValidator.validate(...) before save; Item, Uom, User and
  the plug-in tables don't yet. Promoting the validator to a JPA
  PrePersist/PreUpdate listener attached to a marker interface
  HasExt is the right next step but is its own focused chunk.
* Reference target existence check. FieldType.Reference passes
  through unchanged because the cross-PBC EntityRegistry that would
  let the validator look up "does an instance with this id exist?"
  is a separate seam landing later.
* The customization UI (Tier 1 self-service add a field via the SPA)
  is P3.3 — the runtime enforcement is done, the editor is not.
* Custom-field-aware permissions. The `pii: true` flag is in the
  metadata but not yet read by anything; the DSAR/erasure pipeline
  that will consume it is post-v1.0.