feat: replace HTML export with SvelteKit viewer

Replace the minijinja-based HTML export with a full SvelteKit static
viewer app. The new export produces a ZIP with:
- Pre-built viewer assets (index.html + JS/CSS bundle)
- data.json with all posts, comments, tags, and like counts
- Processed media: 400px thumbnails for grid, full images (2000px
  cap if >5MB), video thumbnails via ffmpeg

Remove minijinja dependency, add include_dir to embed viewer assets
at compile time. Update Dockerfile to copy static/ for builds.

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

backend/Cargo.lock

@@ -896,8 +896,8 @@ dependencies = [
  "dotenvy",
  "futures",
  "image",
+ "include_dir",
  "jsonwebtoken",
- "minijinja",
  "oxipng",
  "rand 0.9.2",
  "serde",
@@ -1577,6 +1577,25 @@ version = "1.12.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e7c5cedc30da3a610cac6b4ba17597bdf7152cf974e8aab3afb3d54455e371c8"
 
+[[package]]
+name = "include_dir"
+version = "0.7.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "923d117408f1e49d914f1a379a309cffe4f18c05cf4e3d12e613a15fc81bd0dd"
+dependencies = [
+ "include_dir_macros",
+]
+
+[[package]]
+name = "include_dir_macros"
+version = "0.7.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7cab85a7ed0bd5f0e76d93846e0147172bed2e2d3f859bcc33a8d9699cad1a75"
+dependencies = [
+ "proc-macro2",
+ "quote",
+]
+
 [[package]]
 name = "indexmap"
 version = "2.13.0"
@@ -1832,12 +1851,6 @@ version = "2.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f8ca58f447f06ed17d5fc4043ce1b10dd205e060fb3ce5b979b8ed8e59ff3f79"
 
-[[package]]
-name = "memo-map"
-version = "0.3.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "38d1115007560874e373613744c6fba374c17688327a71c1476d1a5954cc857b"
-
 [[package]]
 name = "mime"
 version = "0.3.17"
@@ -1854,16 +1867,6 @@ dependencies = [
  "unicase",
 ]
 
-[[package]]
-name = "minijinja"
-version = "2.18.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "328251e58ad8e415be6198888fc207502727dc77945806421ab34f35bf012e7d"
-dependencies = [
- "memo-map",
- "serde",
-]
-
 [[package]]
 name = "miniz_oxide"
 version = "0.8.9"