fix: serve real thumbnails for local photos instead of full-resolution originals (#822)

Add thumbnailService that lazy-generates a WebP thumbnail (800px max, q80) on
first GET /api/photos/:id/thumbnail request using sharp. The generated file is
stored at uploads/journey/thumbs/<sha1>.webp and the path is persisted to
trek_photos.thumbnail_path so subsequent requests are served directly from disk.
Also populates width/height as a side-effect.

streamPhoto now branches on kind for local file_path rows — thumbnail requests
use the stored/generated thumb path; original requests (and fallback when thumb
generation fails) continue to serve the full file. Remote providers (Immich,
Synology) are unaffected.

server/src/services/memories/thumbnailService.ts

@@ -0,0 +1,40 @@
+import sharp from 'sharp'
+import path from 'path'
+import fs from 'fs/promises'
+import crypto from 'crypto'
+
+const THUMB_MAX = 800
+const THUMB_QUALITY = 80
+
+export async function ensureLocalThumbnail(
+  uploadsRoot: string,
+  originalRelPath: string,
+): Promise<{ thumbnailRelPath: string; width: number; height: number } | null> {
+  const originalAbs = path.join(uploadsRoot, originalRelPath)
+  try { await fs.access(originalAbs) } catch { return null }
+
+  // Deterministic name so concurrent requests don't race on the same photo.
+  const hash = crypto.createHash('sha1').update(originalRelPath).digest('hex').slice(0, 16)
+  const thumbRel = `journey/thumbs/${hash}.webp`
+  const thumbAbs = path.join(uploadsRoot, thumbRel)
+
+  try {
+    const [srcStat, dstStat] = await Promise.all([
+      fs.stat(originalAbs),
+      fs.stat(thumbAbs).catch(() => null),
+    ])
+    if (dstStat && dstStat.mtimeMs >= srcStat.mtimeMs) {
+      const meta = await sharp(thumbAbs).metadata()
+      return { thumbnailRelPath: thumbRel, width: meta.width ?? 0, height: meta.height ?? 0 }
+    }
+  } catch { /* regenerate */ }
+
+  await fs.mkdir(path.dirname(thumbAbs), { recursive: true })
+  await sharp(originalAbs)
+    .rotate()
+    .resize({ width: THUMB_MAX, height: THUMB_MAX, fit: 'inside', withoutEnlargement: true })
+    .webp({ quality: THUMB_QUALITY })
+    .toFile(thumbAbs)
+  const meta = await sharp(thumbAbs).metadata()
+  return { thumbnailRelPath: thumbRel, width: meta.width ?? 0, height: meta.height ?? 0 }
+}