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/websocket/client.go

@@ -12,6 +12,8 @@ import (
 
 	"github.com/coder/websocket"
 	"golang.org/x/time/rate"
+
+	"git.avsdev.uk/Go/job-search/internal/util"
 )
 
 // clientSendBuffer is how many messages are queued for a client before it
@@ -26,7 +28,9 @@ const logMessageLimit = 1 << 10
 // *websocket.Conn, or a tcpConn for plain TCP clients (see tcp.go).
 type msgConn interface {
 	Read(ctx context.Context) (websocket.MessageType, []byte, error)
-	Write(ctx context.Context, messageType websocket.MessageType, payload []byte) error
+	Write(
+		ctx context.Context, messageType websocket.MessageType, payload []byte,
+	) error
 	Ping(ctx context.Context) error
 	Close(code websocket.StatusCode, reason string) error
 	CloseNow() error
@@ -49,7 +53,7 @@ type client struct {
 
 // errUnsupportedData is returned by readLoop (and a tcpConn's Write) for a
 // binary message.
-var errUnsupportedData = errors.New("binary frames are not supported")
+var errUnsupportedData = errors.New("websocket: binary frames are not supported")
 
 // newClient creates a client for conn. cancel cancels the context the client
 // runs with; the client calls it when it finishes or is closed. The client's
@@ -59,10 +63,7 @@ func newClient(
 ) *client {
 	limiter := rate.NewLimiter(rate.Inf, 0)
 	if rule := hub.conf.MessageRateLimit; rule.Enabled && rule.PerSecond > 0 {
-		limiter = rate.NewLimiter(
-			rate.Limit(rule.PerSecond),
-			max(rule.Burst, 1),
-		)
+		limiter = rate.NewLimiter(rate.Limit(rule.PerSecond), max(rule.Burst, 1))
 	}
 
 	client := &client{
@@ -95,7 +96,7 @@ func (c *client) run(ctx context.Context) {
 
 	c.sendEnvelope(Envelope{
 		Type: TypeWelcome,
-		Payload: mustRaw(map[string]any{
+		Payload: rawJSON(map[string]any{
 			"clientId": c.id,
 		}),
 	})
@@ -129,16 +130,20 @@ func (c *client) readLoop(ctx context.Context) error {
 		if err != nil {
 			return err
 		}
+
 		c.logMessage(ctx, "message received", data)
 		c.lastSeen.Store(time.Now().UnixNano())
+
 		if messageType != websocket.MessageText {
 			return errUnsupportedData
 		}
+
 		// Check if the client is rate limited for early discard
 		if !c.limiter.Allow() {
 			c.sendError("", "rate limited")
 			continue
 		}
+
 		// Unpack the envelope and pass to handle to process if valid
 		var envelope Envelope
 		if err := json.Unmarshal(data, &envelope); err != nil {
@@ -157,7 +162,7 @@ func (c *client) handle(envelope Envelope) {
 		c.sendEnvelope(Envelope{
 			Type:    TypePong,
 			ID:      envelope.ID,
-			Payload: mustRaw(map[string]any{"time": time.Now().UTC()}),
+			Payload: rawJSON(map[string]any{"time": time.Now().UTC()}),
 		})
 
 	case TypeEcho:
@@ -180,7 +185,7 @@ func (c *client) writeLoop(ctx context.Context) {
 		case <-ctx.Done():
 			return
 		case msg := <-c.send:
-			writeCtx, cancel := withTimeout(ctx, c.hub.conf.WriteTimeout)
+			writeCtx, cancel := util.WithTimeout(ctx, c.hub.conf.WriteTimeout)
 			err := c.conn.Write(writeCtx, websocket.MessageText, msg)
 			cancel()
 			if err != nil {
@@ -217,7 +222,7 @@ func (c *client) pingLoop(ctx context.Context) {
 			if time.Now().Before(lastSeen.Add(pingInterval)) {
 				continue
 			}
-			pingCtx, cancel := withTimeout(ctx, pongTimeout)
+			pingCtx, cancel := util.WithTimeout(ctx, pongTimeout)
 			err := c.conn.Ping(pingCtx)
 			cancel()
 			if err != nil {
@@ -283,29 +288,19 @@ func (c *client) logMessage(ctx context.Context, event string, data []byte) {
 	if !logger.Enabled(ctx, slog.LevelDebug) {
 		return
 	}
+
 	logged, truncated := data, false
 	if len(logged) > logMessageLimit {
 		logged, truncated = logged[:logMessageLimit], true
 	}
+
 	logger.LogAttrs(
 		ctx,
 		slog.LevelDebug,
-		event,
+		event, //nolint:sloglint // a constant message from the caller
 		slog.String("client", c.id),
 		slog.Int("bytes", len(data)),
 		slog.String("message", string(logged)),
 		slog.Bool("truncated", truncated),
 	)
 }
-
-// withTimeout returns ctx with the timeout applied, or ctx itself if the
-// timeout is 0 or less (no limit).
-func withTimeout(
-	ctx context.Context, timeout time.Duration,
-) (context.Context, context.CancelFunc) {
-	if timeout <= 0 {
-		return ctx, func() {
-		}
-	}
-	return context.WithTimeout(ctx, timeout)
-}