feat(fc2a): add slug and paths utilities

slugify() produces ASCII-only lowercase slugs from arbitrary text (used for
artist slugs and tag slugs). paths.derive_subdir/derive_top_level_artist
extract the destination layout and folder→artist convention from an import
source path. hash_suffixed_name builds IR-style 'stem__hash10.ext' filenames.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_slug.py

@@ -0,0 +1,25 @@
+from backend.app.utils.slug import slugify
+
+
+def test_slugify_basic():
+    assert slugify("Bob Ross") == "bob-ross"
+
+
+def test_slugify_punctuation():
+    assert slugify("hello, world!") == "hello-world"
+
+
+def test_slugify_unicode():
+    assert slugify("naïve café") == "naive-cafe"
+
+
+def test_slugify_empty():
+    assert slugify("") == "untitled"
+
+
+def test_slugify_only_punctuation():
+    assert slugify("!!!") == "untitled"
+
+
+def test_slugify_leading_trailing_spaces():
+    assert slugify("  Alice  ") == "alice"