Fix production wedge: propagate context to Google Calendar API calls

Three .Do() calls in google_calendar.go accepted a ctx parameter but
never chained .Context(ctx) into the actual SDK call, so the existing
global 60s request timeout never reached the blocking network call.
One hung Google Calendar request wedged every DB/session-touching
request path in production for three days (2026-08-01 through
2026-08-04), undetected because /health unconditionally returned 200
throughout.

- Wire .Context(ctx) into GetUpcomingEvents, GetEventsByDateRange, and
  GetCalendarList.
- Bound aggregateData's four external fetches with a per-fetch
  sub-context as defense-in-depth (only effective if the callee
  actually honors ctx -- documented as such, not oversold).
- Make GetUpcomingEvents/GetEventsByDateRange fetch calendars
  concurrently instead of sequentially: a review of this fix caught
  that a shared per-fetch deadline over a sequential loop would starve
  calendars past the first under any real latency, silently caching
  partial results as complete. Concurrent fetches give every calendar
  an equal shot at the same deadline instead.
- /health now does a real PingContext DB check instead of a static
  "ok" (Handler.PingDB, tested for both healthy and closed-DB cases).
- Add internal/api/context_audit_test.go: an AST-based structural
  guard that fails any future .Do() call in google_*.go missing
  .Context(...) anywhere in its chain, so this class of bug can't
  silently recur. Verified by deliberately reintroducing the original
  bug against a backup and confirming the guard catches it.
- Add scripts/health-watchdog.sh: cron job restarts the service if
  /health fails twice in a row, five minutes apart.

go test ./... -race is green. Deployed and live-verified.

1 files changed, 51 insertions, 14 deletions

diff --git a/internal/api/google_calendar.go b/internal/api/google_calendar.go
index f441a45..44457bf 100644
--- a/internal/api/google_calendar.go
+++ b/internal/api/google_calendar.go
@@ -6,6 +6,7 @@ import (
 	"log"
 	"sort"
 	"strings"
+	"sync"
 	"time"
 
 	"task-dashboard/internal/models"
@@ -117,19 +118,19 @@ func NewGoogleCalendarClient(ctx context.Context, credentialsFile, calendarIDs,
 
 func (c *GoogleCalendarClient) GetUpcomingEvents(ctx context.Context, maxResults int) ([]models.CalendarEvent, error) {
 	t := time.Now().Format(time.RFC3339)
-	var allEvents []models.CalendarEvent
 
-	for _, calendarID := range c.calendarIDs {
+	fetchOne := func(calendarID string) []models.CalendarEvent {
 		events, err := c.srv.Events.List(calendarID).ShowDeleted(false).
-			SingleEvents(true).TimeMin(t).MaxResults(int64(maxResults)).OrderBy("startTime").Do()
+			SingleEvents(true).TimeMin(t).MaxResults(int64(maxResults)).OrderBy("startTime").
+			Context(ctx).Do()
 		if err != nil {
 			log.Printf("Warning: failed to fetch events from calendar %s: %v", calendarID, err)
-			continue
+			return nil
 		}
-
+		parsed := make([]models.CalendarEvent, 0, len(events.Items))
 		for _, item := range events.Items {
 			start, end := c.parseEventTime(item)
-			allEvents = append(allEvents, models.CalendarEvent{
+			parsed = append(parsed, models.CalendarEvent{
 				ID:               item.Id,
 				Summary:          item.Summary,
 				Description:      item.Description,
@@ -139,8 +140,11 @@ func (c *GoogleCalendarClient) GetUpcomingEvents(ctx context.Context, maxResults
 				RecurringEventID: item.RecurringEventId,
 			})
 		}
+		return parsed
 	}
 
+	allEvents := c.fetchAllCalendars(fetchOne)
+
 	uniqueEvents := deduplicateEvents(allEvents)
 	if len(uniqueEvents) > maxResults {
 		uniqueEvents = uniqueEvents[:maxResults]
@@ -148,22 +152,54 @@ func (c *GoogleCalendarClient) GetUpcomingEvents(ctx context.Context, maxResults
 	return uniqueEvents, nil
 }
 
+// fetchAllCalendars runs fetchOne for every configured calendar concurrently
+// and merges the results. This must be concurrent, not sequential: every
+// caller shares a single deadline-bound ctx (see aggregateData's per-fetch
+// context.WithTimeout in internal/handlers/handlers.go), and a sequential
+// loop would let an early calendar's slow response starve the deadline
+// budget for calendars later in the list -- exactly the failure mode this
+// function exists to prevent (see incident notes 2026-08-04). Running them
+// concurrently means every calendar gets an equal shot at the same shared
+// deadline instead of eating into the next one's share of it.
+func (c *GoogleCalendarClient) fetchAllCalendars(fetchOne func(calendarID string) []models.CalendarEvent) []models.CalendarEvent {
+	var wg sync.WaitGroup
+	var mu sync.Mutex
+	var allEvents []models.CalendarEvent
+
+	for _, calendarID := range c.calendarIDs {
+		wg.Add(1)
+		go func(calendarID string) {
+			defer wg.Done()
+			events := fetchOne(calendarID)
+			if len(events) == 0 {
+				return
+			}
+			mu.Lock()
+			allEvents = append(allEvents, events...)
+			mu.Unlock()
+		}(calendarID)
+	}
+	wg.Wait()
+
+	return allEvents
+}
+
 func (c *GoogleCalendarClient) GetEventsByDateRange(ctx context.Context, start, end time.Time) ([]models.CalendarEvent, error) {
 	timeMin := start.Format(time.RFC3339)
 	timeMax := end.Format(time.RFC3339)
-	var allEvents []models.CalendarEvent
 
-	for _, calendarID := range c.calendarIDs {
+	fetchOne := func(calendarID string) []models.CalendarEvent {
 		events, err := c.srv.Events.List(calendarID).ShowDeleted(false).
-			SingleEvents(true).TimeMin(timeMin).TimeMax(timeMax).OrderBy("startTime").Do()
+			SingleEvents(true).TimeMin(timeMin).TimeMax(timeMax).OrderBy("startTime").
+			Context(ctx).Do()
 		if err != nil {
 			log.Printf("Warning: failed to fetch events from calendar %s: %v", calendarID, err)
-			continue
+			return nil
 		}
-
+		parsed := make([]models.CalendarEvent, 0, len(events.Items))
 		for _, item := range events.Items {
 			evtStart, evtEnd := c.parseEventTime(item)
-			allEvents = append(allEvents, models.CalendarEvent{
+			parsed = append(parsed, models.CalendarEvent{
 				ID:               item.Id,
 				Summary:          item.Summary,
 				Description:      item.Description,
@@ -173,9 +209,10 @@ func (c *GoogleCalendarClient) GetEventsByDateRange(ctx context.Context, start,
 				RecurringEventID: item.RecurringEventId,
 			})
 		}
+		return parsed
 	}
 
-	return deduplicateEvents(allEvents), nil
+	return deduplicateEvents(c.fetchAllCalendars(fetchOne)), nil
 }
 
 // SetCalendarIDs updates the calendar IDs used by the client
@@ -185,7 +222,7 @@ func (c *GoogleCalendarClient) SetCalendarIDs(ids []string) {
 
 // GetCalendarList returns all calendars accessible to the user
 func (c *GoogleCalendarClient) GetCalendarList(ctx context.Context) ([]models.CalendarInfo, error) {
-	list, err := c.srv.CalendarList.List().Do()
+	list, err := c.srv.CalendarList.List().Context(ctx).Do()
 	if err != nil {
 		return nil, fmt.Errorf("failed to fetch calendar list: %w", err)
 	}