refactor(store): propagate context.Context through all Store methods

Every Store interface method (except Close) now takes context.Context
as first parameter. All 54 db.Query/Exec/QueryRow calls in SQLStore
replaced with their *Context variants. DB operations now respect
cancellation and deadlines.

Context sources by caller:
- Engine dbWriter/poll/pruner: engine ctx from Start()
- HTTP handlers: r.Context()
- config.Apply/Export: caller-provided ctx
- TUI/main.go init: context.Background()

RunCheck and all sub-checks (HTTP/ping/port/DNS) accept parent ctx.
HTTP checks now inherit shutdown cancellation instead of rooting in
context.Background(). dbWrite.exec takes ctx so the writer goroutine
can cancel stuck DB operations.

DeleteSite/ImportData use BeginTx(ctx) instead of Begin().

internal/config/export.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"context"
 	"fmt"
 	"os"
 	"sort"
@@ -11,13 +12,13 @@ import (
 	"gopkg.in/yaml.v3"
 )
 
-func Export(s store.Store) (*File, error) {
-	dbAlerts, err := s.GetAllAlerts()
+func Export(ctx context.Context, s store.Store) (*File, error) {
+	dbAlerts, err := s.GetAllAlerts(ctx)
 	if err != nil {
 		return nil, fmt.Errorf("load alerts: %w", err)
 	}
 
-	dbSites, err := s.GetSites()
+	dbSites, err := s.GetSites(ctx)
 	if err != nil {
 		return nil, fmt.Errorf("load sites: %w", err)
 	}