feat(thumbnails): WebP output with Accept content negotiation

Thumbnails are now generated eagerly as lossy WebP (the primary codec) and
served to clients that advertise `Accept: image/webp`; JPEG is kept as a lazy
fallback for older clients and NextCloud, generated on first request and then
cached like WebP.

- ThumbnailFormat{Webp,Jpeg} enum threaded through encode/render/generate, the
  on-disk path ({hash}.webp / {hash}.jpg), the moka cache key
  (file_id, size, format), and cleanup (both formats removed).
- file_handler: parse Accept -> format, format-keyed ETag, `Vary: Accept` on
  every response (incl. 304) so shared caches never serve the wrong codec;
  Content-Type is byte-sniffed (infer) so it always matches the bytes.
- preview_handler (NextCloud) pins JPEG.
- webp = "0.3" (vendored libwebp via cc, no system dependency).

WEBP_QUALITY=82, chosen via a quality sweep (bench Table E1): SSIM within
~0.005 of JPEG q80 (imperceptible at thumbnail scale) for ~62% fewer bytes. On
the photo-realistic bench corpus the full set (3 sizes x 3 photos) drops 65.6%
(213->73 KB); real photos with edges/text land nearer ~25-40%. Encode is +5ms,
paid once in the eager background generator (off the request path).

The bench corpus is now photo-realistic (per-channel sums of low-frequency
sinusoids) instead of white noise, which had distorted codec byte ratios.
Methodology + numbers in benches/WEBP.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/infrastructure/services/thumbnail_service_test.rs

@@ -3,6 +3,8 @@ use std::time::Duration;
 
 use bytes::Bytes;
 
+use crate::application::ports::thumbnail_ports::ThumbnailFormat;
+
 use super::thumbnail_service::{ThumbnailService, ThumbnailSize};
 
 /// Minimal valid 1x1 red PNG (68 bytes).
@@ -46,6 +48,7 @@ async fn generate_thumbnail_from_blob_path() {
             "test-file-id",
             "ab1234567890",
             ThumbnailSize::Icon,
+            ThumbnailFormat::Jpeg,
             &blob_path,
         )
         .await;
@@ -78,6 +81,7 @@ async fn generate_thumbnail_nonexistent_path_returns_error() {
             "missing-id",
             "nonexistent-hash",
             ThumbnailSize::Icon,
+            ThumbnailFormat::Jpeg,
             &bad_path,
         )
         .await;
@@ -105,6 +109,7 @@ async fn generate_thumbnail_from_blob_bytes() {
             "bytes-file-id",
             "bytes-hash-123",
             ThumbnailSize::Preview,
+            ThumbnailFormat::Jpeg,
             Bytes::from(tiny_png()),
         )
         .await;