fix: document exported API, fix naming, and harden Downloadv1.3.3

The remaining golangci-lint findings. These land together because the
Url -> URL rename spans util.go, parsers.go and wrapper.go, and splitting
it would leave an intermediate commit that does not compile.

Download() carried the most serious bug here: try() only prints an error,
it does not return, so a failed request fell through to
resp.Body.Close() on a nil resp and panicked. Every failure path now
returns the zero Downloaded, and callers check Status. ReturnHTTPError
guards against the resulting status 0, which would otherwise panic
WriteHeader. Requests carry a context with a timeout (noctx), and a
download-proxy now routes through ProxiedTransport so it keeps the DA
throttle and timeouts.

Also:
- doc comments on all 48 exported symbols (revive's exported rule, with
  checkPrivateReceivers, since most of app is exported methods on the
  unexported skunkyart type), plus package docs in new doc.go files so
  both the embed and non-embed builds are covered.
- ST1003 naming: UrlBuilder -> URLBuilder, id_search -> idSearch,
  cache_config -> cacheConfig, TXT_RAW -> TxtRaw, mediaUrl -> mediaURL.
- explicit json tags on structs that are unmarshaled (musttag); the
  hyphenated keys already had tags, the rest relied on case-insensitive
  fallback. Behaviour is unchanged.
- modernization: range-over-int, WaitGroup.Go, stale +build lines,
  interface{} -> any, strings.Builder over string concatenation in a loop.

1 files changed, 23 insertions, 15 deletions

diff --git a/app/config.go b/app/config.go
index acd9b71..3b85527 100755
--- a/app/config.go
+++ b/app/config.go
@@ -11,36 +11,41 @@ import (
 	"github.com/zerolabsco/devianter"
 )
 
+// Release carries the build's version and description, set at link time and
+// shown by --help and the API.
 var Release struct {
 	Version     string
 	Description string
 }
 
-type cache_config struct {
-	Enabled        bool
-	MemCache       bool `json:"memcache"`
-	Path           string
-	MaxSize        int64 `json:"max-size"`
-	Lifetime       string
-	UpdateInterval int64 `json:"update-interval"`
+type cacheConfig struct {
+	Enabled        bool   `json:"enabled"`
+	MemCache       bool   `json:"memcache"`
+	Path           string `json:"path"`
+	MaxSize        int64  `json:"max-size"`
+	Lifetime       string `json:"lifetime"`
+	UpdateInterval int64  `json:"update-interval"`
 }
 
 type config struct {
 	cfg           string
-	Listen        string
-	URI           string `json:"uri"`
-	Cache         cache_config
-	Proxy, Nsfw   bool
-	UserAgent     string `json:"user-agent"`
-	DownloadProxy string `json:"download-proxy"`
-	StaticPath    string `json:"static-path"`
+	Listen        string      `json:"listen"`
+	URI           string      `json:"uri"`
+	Cache         cacheConfig `json:"cache"`
+	Proxy         bool        `json:"proxy"`
+	Nsfw          bool        `json:"nsfw"`
+	UserAgent     string      `json:"user-agent"`
+	DownloadProxy string      `json:"download-proxy"`
+	StaticPath    string      `json:"static-path"`
 }
 
+// CFG is the running instance's configuration, holding the defaults below until
+// ExecuteConfig overwrites them from the config file.
 var CFG = config{
 	cfg:    "config.json",
 	Listen: "127.0.0.1:3003",
 	URI:    "/",
-	Cache: cache_config{
+	Cache: cacheConfig{
 		Enabled:        false,
 		Path:           "cache",
 		UpdateInterval: 1,
@@ -53,6 +58,9 @@ var CFG = config{
 
 var lifetimeParsed int64
 
+// ExecuteConfig loads the config file into CFG, validates it, and starts the
+// cache rotation loop if caching is on. It exits the process on a config that
+// cannot be read or that asks for caching without proxying.
 func ExecuteConfig() {
 	if CFG.cfg != "" {
 		f, err := os.ReadFile(CFG.cfg)