Address remaining QA items: index, validation, accessibility, guard

- S1: Add composite index (is_active, is_dismissed, remind_at) for
  /due query performance with multi-user scaling
- W3: Snooze endpoint rejects dismissed/inactive reminders (409)
- W4: Custom field_validator on ReminderSnooze for clear error message
- S2: aria-label on all snooze/dismiss buttons in banner and toasts

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/alembic/versions/017_add_reminder_snoozed_until.py

@@ -18,7 +18,13 @@ depends_on = None
 
 def upgrade() -> None:
     op.add_column("reminders", sa.Column("snoozed_until", sa.DateTime(), nullable=True))
+    op.create_index(
+        "ix_reminders_due_lookup",
+        "reminders",
+        ["is_active", "is_dismissed", "remind_at"],
+    )
 
 
 def downgrade() -> None:
+    op.drop_index("ix_reminders_due_lookup", table_name="reminders")
     op.drop_column("reminders", "snoozed_until")

backend/app/routers/reminders.py

@@ -79,6 +79,9 @@ async def snooze_reminder(
     if not reminder:
         raise HTTPException(status_code=404, detail="Reminder not found")
 
+    if reminder.is_dismissed or not reminder.is_active:
+        raise HTTPException(status_code=409, detail="Cannot snooze a dismissed or inactive reminder")
+
     reminder.snoozed_until = datetime.now() + timedelta(minutes=body.minutes)
 
     await db.commit()

backend/app/schemas/reminder.py

@@ -1,4 +1,4 @@
-from pydantic import BaseModel, ConfigDict
+from pydantic import BaseModel, ConfigDict, field_validator
 from datetime import datetime
 from typing import Literal, Optional
 
@@ -23,6 +23,13 @@ class ReminderUpdate(BaseModel):
 class ReminderSnooze(BaseModel):
     minutes: Literal[5, 10, 15]
 
+    @field_validator('minutes', mode='before')
+    @classmethod
+    def validate_minutes(cls, v: int) -> int:
+        if v not in (5, 10, 15):
+            raise ValueError('Snooze duration must be 5, 10, or 15 minutes')
+        return v
+
 
 class ReminderResponse(BaseModel):
     id: int

frontend/src/components/dashboard/AlertBanner.tsx

@@ -38,6 +38,7 @@ export default function AlertBanner({ alerts, onDismiss, onSnooze }: AlertBanner
                 <button
                   key={m}
                   onClick={() => onSnooze(alert.id, m)}
+                  aria-label={`Snooze "${alert.title}" for ${m} minutes`}
                   className="px-1.5 py-0.5 rounded bg-secondary hover:bg-accent/10 hover:text-accent text-muted-foreground transition-colors"
                 >
                   {m}m
@@ -46,6 +47,7 @@ export default function AlertBanner({ alerts, onDismiss, onSnooze }: AlertBanner
             </div>
             <button
               onClick={() => onDismiss(alert.id)}
+              aria-label={`Dismiss "${alert.title}"`}
               className="p-1 rounded hover:bg-accent/10 hover:text-accent text-muted-foreground transition-colors shrink-0"
             >
               <X className="h-3.5 w-3.5" />

frontend/src/hooks/useAlerts.tsx

@@ -108,6 +108,7 @@ export function AlertsProvider({ children }: { children: ReactNode }) {
                 <button
                   key={m}
                   onClick={() => snoozeRef.current(reminder.id, m as 5 | 10 | 15)}
+                  aria-label={`Snooze "${reminder.title}" for ${m} minutes`}
                   className="px-1.5 py-0.5 rounded bg-secondary hover:bg-card-elevated text-muted-foreground hover:text-foreground transition-colors"
                 >
                   {m}m
@@ -116,6 +117,7 @@ export function AlertsProvider({ children }: { children: ReactNode }) {
             </div>
             <button
               onClick={() => dismissRef.current(reminder.id)}
+              aria-label={`Dismiss "${reminder.title}"`}
               className="ml-auto px-2 py-0.5 rounded text-[11px] bg-secondary hover:bg-card-elevated text-muted-foreground hover:text-foreground transition-colors"
             >
               Dismiss