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_users.go

@@ -1,6 +1,7 @@
 package tui
 
 import (
+	"context"
 	"fmt"
 
 	tea "github.com/charmbracelet/bubbletea"
@@ -118,10 +119,10 @@ func (m *Model) submitUserForm() tea.Cmd {
 	m.state = stateUsers
 	if id > 0 {
 		return writeCmd("Update user", func() error {
-			return st.UpdateUser(id, username, key, role)
+			return st.UpdateUser(context.Background(), id, username, key, role)
 		})
 	}
 	return writeCmd("Add user", func() error {
-		return st.AddUser(username, key, role)
+		return st.AddUser(context.Background(), username, key, role)
 	})
 }