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/store/migrate.go

@@ -17,21 +17,6 @@ import (
 //go:embed migrations/*.sql
 var migrationsFS embed.FS
 
-// ErrNeedsMigration is returned by Open when the database has pending
-// migrations. Apply them with the migrate command, or pass
-// WithAutoMigrate.
-var ErrNeedsMigration = errors.New("store: database needs migrating")
-
-// ErrSchemaTooNew is returned by Open when the database has migrations
-// this build doesn't know about, i.e. it was migrated by a newer build.
-var ErrSchemaTooNew = errors.New("store: database schema is newer than this build")
-
-// ErrWouldEmptyDatabase is returned by StepDown when the migration would drop
-// all of the tables and thus empty the database.
-var ErrWouldEmptyDatabase = errors.New(
-	"migration cancelled; database would be emptied by undoing all migrations",
-)
-
 // NewMigrator returns a goose Provider over the store's embedded migrations,
 // for applying, rolling back or inspecting them. Each migration runs in its
 // own transaction, so a failed migration leaves the database at the previous
@@ -96,13 +81,15 @@ func checkNotNewer(ctx context.Context, provider *goose.Provider) error {
 	if err != nil {
 		return err
 	}
+
 	if latest := LatestVersion(provider); current > latest {
 		return fmt.Errorf(
-			"database is at version %d but this build only knows up to %d; use a newer build",
+			"store: database is at version %d but this build only knows up to %d; use a newer build",
 			current,
 			latest,
 		)
 	}
+
 	return nil
 }
 
@@ -116,7 +103,7 @@ func prepareSchema(ctx context.Context, db *sql.DB, opts options) error {
 
 	if opts.autoMigrate {
 		if _, err := provider.Up(ctx); err != nil {
-			return fmt.Errorf("store: migrating database: %w", err)
+			return fmt.Errorf("migrating database: %w", err)
 		}
 	}
 
@@ -184,6 +171,7 @@ func StepDown(
 			applied++
 		}
 	}
+
 	all := steps == 0 || steps >= applied
 	if all && !confirmDrop {
 		return results, ErrWouldEmptyDatabase