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/tui/tui.go

@@ -1,6 +1,7 @@
 package tui
 
 import (
+	"context"
 	"os"
 	"time"
 
@@ -180,7 +181,7 @@ func InitialModel(isAdmin bool, s store.Store, eng *monitor.Engine, version stri
 	spring := harmonica.NewSpring(harmonica.FPS(10), 6.0, 0.4)
 	collapsed := loadCollapsed(s)
 
-	themeName, _ := s.GetPreference("theme")
+	themeName, _ := s.GetPreference(context.Background(), "theme")
 	theme := themeByName(themeName)
 	themeIdx := 0
 	for i, t := range themes {