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/cluster/probe.go

@@ -152,7 +152,7 @@ loop:
 			defer wg.Done()
 			defer func() { <-sem }()
 
-			cr := monitor.RunCheck(s, strict, insecure, false, allowPrivate)
+			cr := monitor.RunCheck(ctx, s, strict, insecure, false, allowPrivate)
 			mu.Lock()
 			results = append(results, probeResultItem{
 				SiteID:      s.ID,