Added fetch service and a mock Find a Tender server

Fetching:
- Add `fetch service`, which runs a worker per enabled source until
  SIGINT/SIGTERM (internal/fetcher)
- Add util.Worker: runs a task on an interval with a since-checkpoint
  that only advances on success, and a shutdown timeout for in-flight
  work; plus AwaitTask/AwaitTaskE for calls that ignore cancellation
- Add fts_mock and sleeper sources
- Sleeper: sleeps periodically to emulate work being done. Can be
  instructed to block the shutdown signal or handle it cleanly

Mock FTS server (cmd/mock-fts, internal/mock/findATender):
- Serves GET /api/1.0/ocdsReleasePackages with FTS paging, query
  validation, UK local time handling and per-client rate limiting
- Generates fictional releases from an embedded catalog, moving
  procedures through planning, tender and award, and keeps them in a
  JSON Lines store that survives restarts
- Add logging.AccessLog for combined/common access logs

internal/cli/fetch/command.go

@@ -19,5 +19,6 @@ func NewCommand(opts *cli.Options) *cobra.Command {
 		},
 	}
 	cmd.AddCommand(commandOnce(opts))
+	cmd.AddCommand(commandService(opts))
 	return cmd
 }

internal/cli/fetch/service.go

@@ -0,0 +1,70 @@
+package fetch
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"git.avsdev.uk/Go/job-search/internal/cli"
+	"git.avsdev.uk/Go/job-search/internal/fetcher"
+	"github.com/spf13/cobra"
+)
+
+func commandService(opts *cli.Options) *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "service",
+		Short: "Fetch current opportunities - background service",
+		PersistentPreRun: func(cmd *cobra.Command, args []string) {
+			slog.Debug("fetch.service.PersistentPreRun")
+		},
+		PersistentPostRun: func(cmd *cobra.Command, args []string) {
+			slog.Debug("fetch.service.PersistentPostRun")
+		},
+		RunE: func(cmd *cobra.Command, args []string) error {
+			slog.Debug("fetch.service.Run")
+
+			foreground, _ := cmd.Flags().GetBool("foreground")
+
+			conf, err := opts.RunSetup(cmd, true, foreground)
+			if err != nil {
+				return fmt.Errorf("no configuration loaded (%w)", err)
+			}
+
+			since, sinceErr := cmd.Flags().GetTime("since")
+			if sinceErr != nil {
+				return sinceErr
+			}
+
+			// ctx is cancelled on the first SIGINT (Ctrl+C) or SIGTERM.
+			ctx, stop := signal.NotifyContext(
+				context.Background(), os.Interrupt, syscall.SIGTERM,
+			)
+			defer stop()
+
+			// After the first signal, restore default signal handling so a second
+			// Ctrl+C force-kills the process if graceful shutdown hangs.
+			go func() {
+				<-ctx.Done()
+				stop()
+			}()
+
+			fetcher.Run(ctx, conf, since)
+
+			return nil
+		},
+	}
+	cmd.Flags().Time(
+		"since",
+		time.Time{},
+		[]string{
+			time.DateOnly, time.DateTime,
+		},
+		"contract updates published since YYYY-MM-DD[ hh:mm:ss]",
+	)
+	cmd.Flags().Bool("foreground", false, "run in the foreground")
+	return cmd
+}