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/tab_sites.go

@@ -1,6 +1,7 @@
 package tui
 
 import (
+	"context"
 	"fmt"
 	"net/url"
 	"strconv"
@@ -562,7 +563,7 @@ func (m *Model) submitSiteForm() tea.Cmd {
 		// follows in the Cmd. New sites enter the engine via its poll loop
 		// once the insert lands.
 		m.engine.UpdateSiteConfig(site)
-		return writeCmd("Update site", func() error { return st.UpdateSite(site) })
+		return writeCmd("Update site", func() error { return st.UpdateSite(context.Background(), site) })
 	}
-	return writeCmd("Add site", func() error { return st.AddSite(site) })
+	return writeCmd("Add site", func() error { return st.AddSite(context.Background(), site) })
 }