Added CODE_CONVENTIONS.md and linters to enforce it, and applied it across the codebase

CODE_CONVENTIONS.md sets out the conventions for Go code in this
repository: naming, layout, comments, errors, logging, configuration and
tests. .golangci.json enables the linters that check the parts that can be
checked (each limited to the rules the conventions use); the rest is
applied by hand.

Applied across the codebase:
- Naming: full-word names in application code, initialisms in one case
  (BuyerID, PrintJSON, PrintYAML, ocdsGetBuyerID), no names shadowing
  built-ins (min/max are minGap/maxGap), mustRaw renamed rawJSON as it
  returns nil rather than panicking, and config.ConfigLevelToLogLevel
  renamed LevelToLogLevel.
- Layout: lines kept to 80 characters where possible, assignments taken
  out of if and for statements that don't fit, wrapped arguments all on
  one line or one per line, steps separated by blank lines, no else after
  a return, function and struct literals wrapped.
- Comments: doc comments on every exported name (including each section's
  Map, Flatten, TypeOf, ValueOf and SetValue), section banners, British
  spelling.
- Errors: a created error starts with its package name, a wrapping error
  adds only what was being done, sentinels in one commented var block,
  errors.Is and errors.As in place of == and type assertions, and no
  err := shadowing of a named result. That fixes the mock server losing
  its access log's close error.
- Logging: the fetcher passes its workers an untagged logger, so
  findatender's lines are tagged with their own component rather than
  with fetcher's as well; cmd/mock-fts logs "stopped" with slog.
- Configuration: util.WithTimeout (moved from the websocket package)
  applies a timeout of 0 or less as no limit, used for the websocket
  write and ping, TCP close and tcpserver shutdown timeouts; every setting
  documents what 0 or empty means, including the rate limit rule's keys
  in internal/config/README.md.
- Tests: related tests merged into table-driven ones with named cases.

Separately from CODE_CONVENTIONS.md, internal/store/README.md gains a soft
SQL style convention for the queries in internal/store/queries.go, and the
queries are reformatted to follow it.

internal/config/load.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"errors"
 	"fmt"
 	"log/slog"
 	"os"
@@ -41,13 +42,13 @@ func loadEnv() error {
 	return nil
 }
 
-// loadConfig sets up Viper with the defaults, cfgFile (or config.* found in
+// loadConfig sets up Viper with the defaults, configFile (or config.* found in
 // EtcConfigPath or the working directory) and JOBSEARCH_* environment
 // variables. A missing config file is only a warning.
-func loadConfig(cfgFile string) error {
-	if cfgFile != "" {
+func loadConfig(configFile string) error {
+	if configFile != "" {
 		// Use config file from the flag.
-		viper.SetConfigFile(cfgFile)
+		viper.SetConfigFile(configFile)
 	} else {
 		// Search config in /etc or local directory.
 		viper.AddConfigPath(EtcConfigPath)
@@ -64,26 +65,25 @@ func loadConfig(cfgFile string) error {
 	viper.AutomaticEnv() // read in environment variables that match
 
 	err := viper.ReadInConfig()
-	if err != nil {
-		if _, ok := err.(viper.ConfigFileNotFoundError); ok {
-			// Indicates an explicitly set config file is not found or
-			// that no config file was found in any search path
-			slog.Warn("no config file found; using defaults")
-		} else {
-			// Config file was found but another error was produced
-			return err
-		}
-	} else {
-		// Config file found and successfully parsed
+	var notFound viper.ConfigFileNotFoundError
+	switch {
+	case errors.As(err, &notFound):
+		// No config file was found in any search path.
+		slog.Warn("no config file found; using defaults")
+	case err != nil:
+		// A config file was found but couldn't be read or parsed.
+		return err
+	default:
 		slog.Info("using config file", "file", viper.ConfigFileUsed())
 	}
+
 	return nil
 }
 
 // Init loads the .env file and the configuration (see loadConfig) into conf.
 // A debug or error log level already set in conf, from --debug or --quiet,
 // is kept over the loaded one.
-func Init(cfgFile string, conf *JobSearchConfig) error {
+func Init(configFile string, conf *JobSearchConfig) error {
 	if err := loadEnv(); err != nil {
 		return err
 	}
@@ -91,7 +91,7 @@ func Init(cfgFile string, conf *JobSearchConfig) error {
 		printEnv()
 	}
 
-	if err := loadConfig(cfgFile); err != nil {
+	if err := loadConfig(configFile); err != nil {
 		return err
 	}