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/monitor/history.go

@@ -1,6 +1,9 @@
 package monitor
 
-import "time"
+import (
+	"context"
+	"time"
+)
 
 const maxHistoryLen = 60
 
@@ -12,7 +15,7 @@ type SiteHistory struct {
 }
 
 func (e *Engine) InitHistory() {
-	all, err := e.db.LoadAllHistory(maxHistoryLen)
+	all, err := e.db.LoadAllHistory(context.Background(), maxHistoryLen)
 	if err != nil {
 		e.AddLog("Failed to load check history: " + err.Error())
 		return