feat: cookies desde navegador, fallback watch-page y optimizacion integral del nucleo

Extraccion autenticada:
- import_from_browser (Brave) con fallback CDP headless para cookies app-bound v20
- extract_via_watch_page: GET plano + ytInitialPlayerResponse cuando yt-dlp falla
  con sesion logueada (members-only); regex y opener cacheados
- js_runtimes (node/deno/bun/quickjs) propagado a todos los ydl_opts
- rutas de Brave multiplataforma (Windows/macOS/Linux)

Webapp UX: chips de filtros removibles, skeleton loaders, estado de vista en URL,
memoria de scroll, copyMd/openMd, import de cookies desde navegador, no-cache de statics

Rendimiento:
- entorno Jinja2 cacheado por directorio de plantilla (antes 1 por nota)
- _rank_unranked con guarda (antes full-scan en cada arranque/import)
- upsert_videos con executemany; dashboard sin N+1 (GROUP BY + conteo de tags en SQL)
- thumbnails en paralelo (6 hilos, CDN ytimg); handlers bloqueantes -> def (threadpool)
- reconcile de arranque en hilo daemon: uvicorn arriba al instante (0.95s con 1503 md),
  healthz expone reconcile_done
- Store.transaction(): escrituras por video agrupadas (~6 commits -> 3)

Refactor: helpers unicos (extract_handle->discover, safe_dirname/filename->render,
order_pending->store, keep_ref->config, seconds_to_ts solo en segments);
re-render del CLI delega en pipeline.re_render_videos (retira huerfanos y marca done);
fuera wrappers muertos de segments.py

tests/test_watch_page_fallback.py

@@ -0,0 +1,88 @@
+from __future__ import annotations
+
+import io
+import json
+from pathlib import Path
+
+from yt_scraper.extract import extract_via_watch_page
+
+
+WATCH_PAGE = (
+    "var ytInitialPlayerResponse = "
+    + json.dumps({
+        "playabilityStatus": {"status": "OK"},
+        "videoDetails": {
+            "title": "Members only test",
+            "author": "Canal",
+            "lengthSeconds": "42",
+            "viewCount": "7",
+            "shortDescription": "desc",
+            "keywords": ["a", "b"],
+        },
+        "microformat": {"playerMicroformatRenderer": {"publishDate": "2026-09-01"}},
+        "captions": {"playerCaptionsTracklistRenderer": {"captionTracks": [
+            {"languageCode": "es", "kind": "asr", "baseUrl": "https://captions.test/es"},
+        ]}},
+    })
+    + ";"
+)
+
+JSON3 = json.dumps({
+    "events": [
+        {"tStartMs": 0, "dDurationMs": 1000, "segs": [{"utf8": "hola "}, {"utf8": "mundo"}]},
+        {"tStartMs": 1000, "dDurationMs": 500, "segs": [{"utf8": "segundo"}]},
+    ]
+})
+
+
+class _FakeResponse(io.BytesIO):
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *a):
+        return False
+
+
+def test_watch_page_fallback_builds_segments(tmp_path: Path, monkeypatch):
+    from yt_scraper import extract as ex
+
+    def fake_urlopen(cookies_file, url, **kw):
+        if url.startswith("https://www.youtube.com/"):
+            return _FakeResponse(WATCH_PAGE.encode("utf-8"))
+        assert "fmt=json3" in url
+        return _FakeResponse(JSON3.encode("utf-8"))
+
+    monkeypatch.setattr(ex, "_session_urlopen", fake_urlopen)
+    monkeypatch.setattr(ex.GLOBAL_PACER, "wait", lambda **kw: None)
+
+    data = extract_via_watch_page(
+        "https://www.youtube.com/watch?v=xyz", "cookies/fake.txt", {"es": "any"}
+    )
+
+    assert data is not None
+    assert data.subtitle is not None and data.subtitle.lang == "es"
+    # parse_auto_dump merges caption events with no gap between them.
+    assert len(data.segments) == 1
+    assert data.segments[0].text == "hola mundo segundo"
+    assert data.info["title"] == "Members only test"
+    assert data.info["upload_date"] == "20260901"
+    assert data.info["duration"] == 42
+    assert data.skip_reason is None
+
+
+def test_watch_page_fallback_without_tracks_returns_none(tmp_path: Path, monkeypatch):
+    from yt_scraper import extract as ex
+
+    page = WATCH_PAGE.replace('"captionTracks": [{', '"captionTracks": [{')
+    page = "var ytInitialPlayerResponse = " + json.dumps({
+        "playabilityStatus": {"status": "OK"},
+        "videoDetails": {"title": "x"},
+    }) + ";"
+
+    monkeypatch.setattr(ex, "_session_urlopen",
+                        lambda *a, **kw: _FakeResponse(page.encode("utf-8")))
+    monkeypatch.setattr(ex.GLOBAL_PACER, "wait", lambda **kw: None)
+
+    assert extract_via_watch_page(
+        "https://www.youtube.com/watch?v=xyz", "cookies/fake.txt", {"es": "any"}
+    ) is None