feat: editable taken_at + folder-based date repair and filter

Lets operators fix corrupted capture dates at scale. Adds an editable
Date Taken field with a folder/filename-derived suggestion hint, a bulk
Date Taken section in the multi-select sidebar that either applies one
date to the whole selection or infers a per-photo date from each path,
a warning badge on thumbnails whose stored date disagrees with the
path, and a "Date issues" filter pill so suspicious photos can be
surfaced and fixed as a group. Edits are written back to EXIF on disk
so rescans don't clobber the fix.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/timeline/PhotoThumbnail.tsx

@@ -1,5 +1,13 @@
 import { useState, useEffect, useCallback, useRef } from 'react'
-import { Star, ShoppingBasket, Trash2, RefreshCw, Check, Copy } from 'lucide-react'
+import {
+  Star,
+  ShoppingBasket,
+  Trash2,
+  RefreshCw,
+  Check,
+  Copy,
+  AlertTriangle,
+} from 'lucide-react'
 import clsx from 'clsx'
 import { photos as photosApi } from '../../services/api'
 import type { Photo } from '../../types/photo'
@@ -92,6 +100,13 @@ export function PhotoThumbnail({
   const baseUrl = photosApi.getThumbnailUrl(photo.id, 'medium')
   const thumbnailUrl = retryCount > 0 ? `${baseUrl}?retry=${retryCount}` : baseUrl
 
+  // "Capture date probably wrong" — read straight from the stored
+  // `has_date_warning` flag rather than recomputing the heuristic
+  // client-side. The backend sets this column at scan time and
+  // refreshes it on any taken_at edit, so the UI, the filter, and the
+  // thumbnail badge all read from one source of truth.
+  const dateWarning = photo.has_date_warning === true
+
   // Square cells (Lightroom-style grid). Variable-aspect cells previously
   // overflowed their row because TanStack Virtual estimates row height as a
   // single fixed value — portraits in a landscape row would overlap the row
@@ -323,6 +338,21 @@ export function PhotoThumbnail({
             <Trash2 className={THUMB_BADGE_ICON} strokeWidth={2.5} />
           </div>
         )}
+        {dateWarning && (
+          <div
+            className={clsx(
+              THUMB_BADGE_BASE,
+              THUMB_BADGE_SQUARE,
+              // Amber fill with the same chiseled frame as other affirmative
+              // badges so it reads as a first-class warning rather than a
+              // neutral info chip.
+              'bg-amber-500 shadow-[0_0_0_1px_rgba(0,0,0,0.55),inset_0_1px_0_rgba(255,255,255,0.28),0_1px_2px_rgba(0,0,0,0.5)]'
+            )}
+            title="Capture date may be wrong — folder/filename suggests a different date"
+          >
+            <AlertTriangle className={THUMB_BADGE_ICON} strokeWidth={2.5} />
+          </div>
+        )}
       </div>
 
       {/* TR — file-type metadata (RAW / VIDEO) */}