Add Phase 4: code quality, CI/CD, and formatting

- testthat infrastructure with 15 tests covering env-var guards,
  return types for all format/save functions, and spelling
- inst/WORDLIST with 52 domain terms (LightGBM, MinIO, Parquet, etc.)
- Spelling test wired into devtools::test() via test-spelling.R
- styler::style_file() added as step 0 in deploy.R (auto-fixes before ship)
- .gitea/workflows/test.yaml: runs testthat suite on push
- .gitea/workflows/lint.yaml: lychee link check + styler dry-run on push
- Removed internal IP address from comment in train_production_model()
- Language: en-US added to DESCRIPTION

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

tests/testthat/test-validation.R

@@ -0,0 +1,48 @@
+test_that("connect_baf() errors on missing BAF_ENDPOINT", {
+  withr::with_envvar(
+    c(BAF_ENDPOINT = "", BAF_KEY = "key", BAF_SECRET = "secret", BAF_BUCKET = "baf-fraud"),
+    expect_error(connect_baf("some/prefix"), "BAF_ENDPOINT")
+  )
+})
+
+test_that("connect_baf() errors on missing BAF_KEY", {
+  withr::with_envvar(
+    c(BAF_ENDPOINT = "minio:9000", BAF_KEY = "", BAF_SECRET = "secret", BAF_BUCKET = "baf-fraud"),
+    expect_error(connect_baf("some/prefix"), "BAF_KEY")
+  )
+})
+
+test_that("connect_baf() errors on missing BAF_SECRET", {
+  withr::with_envvar(
+    c(BAF_ENDPOINT = "minio:9000", BAF_KEY = "key", BAF_SECRET = "", BAF_BUCKET = "baf-fraud"),
+    expect_error(connect_baf("some/prefix"), "BAF_SECRET")
+  )
+})
+
+test_that("connect_baf() errors on missing BAF_BUCKET", {
+  withr::with_envvar(
+    c(BAF_ENDPOINT = "minio:9000", BAF_KEY = "key", BAF_SECRET = "secret", BAF_BUCKET = ""),
+    expect_error(connect_baf("some/prefix"), "BAF_BUCKET")
+  )
+})
+
+test_that("convert_to_parquet() errors on missing BAF_ENDPOINT", {
+  withr::with_envvar(
+    c(BAF_ENDPOINT = "", BAF_KEY = "key", BAF_SECRET = "secret"),
+    expect_error(convert_to_parquet("01_raw", "02_intermediate"), "BAF_ENDPOINT")
+  )
+})
+
+test_that("convert_to_parquet() errors on missing BAF_KEY", {
+  withr::with_envvar(
+    c(BAF_ENDPOINT = "minio:9000", BAF_KEY = "", BAF_SECRET = "secret"),
+    expect_error(convert_to_parquet("01_raw", "02_intermediate"), "BAF_KEY")
+  )
+})
+
+test_that("convert_to_parquet() errors on missing BAF_SECRET", {
+  withr::with_envvar(
+    c(BAF_ENDPOINT = "minio:9000", BAF_KEY = "key", BAF_SECRET = ""),
+    expect_error(convert_to_parquet("01_raw", "02_intermediate"), "BAF_SECRET")
+  )
+})