Implement Phases 1-5: skeleton, OAuth, categories, video sync/feed, playback

- FastAPI + PostgreSQL + Alembic + React/Vite skeleton, Docker Compose, healthcheck
- Google OAuth (single allowed account), encrypted refresh token storage
- Subscriptions sync with pagination, uploads playlist batch fetch
- Categories CRUD, many-to-many channel assignment, category filtering
- Video sync (playlistItems + videos.list batching), cached feed with cursor
  pagination, background scheduler (APScheduler)
- Video detail page with YouTube embed player
- SPA fallback routing, optimistic UI updates, client-side query caching

40 backend tests covering OAuth allow-list, sync idempotency, cascade deletes,
cursor pagination, and category filtering.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/core/slugify.py

@@ -0,0 +1,29 @@
+import re
+
+_CYRILLIC_MAP = {
+    "а": "a", "б": "b", "в": "v", "г": "g", "д": "d", "е": "e", "ё": "e",
+    "ж": "zh", "з": "z", "и": "i", "й": "y", "к": "k", "л": "l", "м": "m",
+    "н": "n", "о": "o", "п": "p", "р": "r", "с": "s", "т": "t", "у": "u",
+    "ф": "f", "х": "h", "ц": "ts", "ч": "ch", "ш": "sh", "щ": "sch", "ъ": "",
+    "ы": "y", "ь": "", "э": "e", "ю": "yu", "я": "ya",
+}
+
+
+def _transliterate(text: str) -> str:
+    return "".join(_CYRILLIC_MAP.get(ch, ch) for ch in text)
+
+
+def slugify(text: str) -> str:
+    text = _transliterate(text.lower())
+    text = re.sub(r"[^a-z0-9]+", "-", text).strip("-")
+    return text or "category"
+
+
+def unique_slugify(text: str, existing_slugs: set[str]) -> str:
+    base = slugify(text)
+    if base not in existing_slugs:
+        return base
+    suffix = 2
+    while f"{base}-{suffix}" in existing_slugs:
+        suffix += 1
+    return f"{base}-{suffix}"