Add schema DELETE/update API with gRPC parity, frontend controls, and full test coverage

[Copilot]

Schema templates were read/write-only — there was no way to delete a schema or update it through a documented, tested path. This adds DELETE /schema/{id} alongside PUT update semantics, gRPC equivalents, frontend controls, and complete documentation.

Backend

• pkg/schema — DeleteTemplate(schemaID): removes file from disk, evicts cache entry. Introduces ErrSchemaNotFound sentinel so callers use errors.Is() instead of string matching.

• DELETE /schema/{schemaID} — requires write permission; returns 404 for missing schemas, 400 for invalid IDs (path-traversal safe).

• gRPC Process handler — three new operation strings routed before entity ops:

  operation	Equivalent REST
  SCHEMA_GET	GET /schema/{id}
  SCHEMA_PUT	PUT /schema/{id}
  SCHEMA_DELETE	DELETE /schema/{id}

Frontend

• Templates list: each row gets a 🗑 delete button inline.
• Editor panel: Delete button beside Save; clears editor and refreshes list on success.
• Inline styles replaced with .schema-list-row / .schema-list-btn CSS classes.

API Spec & Docs

• api/openapi.yaml: full DELETE /schema/{schemaID} path with 200/401/403/404.
• docs/usage/api-reference.md: delete endpoint docs + gRPC schema operations table with Go examples.
• docs/usage/schemas.md: new Updating and Deleting sections covering REST, Web UI, and gRPC.
• README.md: DELETE /schema/{schemaID} reference with curl example.

Tests

• TestDeleteTemplate (6 sub-tests): disk/cache removal, not-found, empty ID, path traversal, list exclusion, validate-after-delete.
• TestHandleSchemaDelete (7 sub-tests): auth, success, 404-after-delete, not-found, read-only forbidden, path traversal, list exclusion.
• TestProcessSchemaOperations (6 sub-tests): full gRPC round-trip for all three schema ops.
• 7 new pytest integration tests covering the same surface via live server.

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