refactor(scripts): extract shared sae_* helpers and dedupe gallery builders

Consolidate copy-pasted logic across the gallery/run scripts into shared
modules:
  - sae_env.py     — zero-dependency .env loader (populates os.environ)
  - sae_tmdb.py    — TMDB API helpers (tmdb_get, person images, id lookups)
  - sae_jellyfin.py— Jellyfin API helpers (jf_get, id/URL normalisation)
  - sae_gallery.py — image download + gallery.json writing

make_gallery, make_jellyfin_gallery and filter_gallery now import these
instead of carrying their own near-identical copies.

scripts/sae_env.py

@@ -0,0 +1,49 @@
+"""Zero-dependency .env loader.
+
+The gallery/run scripts read credentials (JELLYFIN_URL, JELLYFIN_API_KEY,
+TMDB_API_KEY) from os.environ, but nothing populated os.environ from the
+project's .env file — so the keys only worked if you'd manually
+`set -a; source .env`. This loads .env into os.environ on import, without
+pulling in python-dotenv.
+
+Import this module early (before argparse reads os.environ defaults) so the
+.env values are available as defaults.
+"""
+
+import os
+from pathlib import Path
+
+# Walk up from this file to find the project root's .env (scripts/ is one
+# level down from the repo root).
+_DEFAULT_ENV = Path(__file__).resolve().parent.parent / ".env"
+
+
+def load_env(path: Path | str = _DEFAULT_ENV, *, override: bool = False) -> None:
+    """Parse a .env file into os.environ.
+
+    Lines are KEY=VALUE; blank lines and #-comments are ignored, surrounding
+    whitespace is stripped, and a single layer of matching quotes around the
+    value is removed. Existing os.environ entries win unless override=True, so
+    an explicitly exported var (or one set on the command line) takes
+    precedence over the file.
+    """
+    path = Path(path)
+    if not path.is_file():
+        return
+    for raw in path.read_text().splitlines():
+        line = raw.strip()
+        if not line or line.startswith("#") or "=" not in line:
+            continue
+        key, _, value = line.partition("=")
+        key = key.strip()
+        value = value.strip()
+        if len(value) >= 2 and value[0] == value[-1] and value[0] in ("'", '"'):
+            value = value[1:-1]
+        if not key:
+            continue
+        if override or key not in os.environ:
+            os.environ[key] = value
+
+
+# Load on import so `os.environ.get(...)` argparse defaults see .env values.
+load_env()