feat: add CIE-10 diagnosticos migration endpoint and table

- migrations/20260717_cie10_diagnosticos.sql: CREATE TABLE cie10_diagnosticos
- api/lab/ingest_diagnosticos.php: POST endpoint that receives batches of
  {cod, concepto} rows and upserts into cie10_diagnosticos (ON DUPLICATE KEY)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

migrations/20260717_cie10_diagnosticos.sql

@@ -0,0 +1,6 @@
+-- Migración: tabla CIE-10 diagnósticos desde Firebird
+CREATE TABLE IF NOT EXISTS cie10_diagnosticos (
+    cod_diag  VARCHAR(10)  NOT NULL,
+    concepto  VARCHAR(500) NOT NULL,
+    PRIMARY KEY (cod_diag)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;