refactor: consolidate _sc_headers into http.scrapecreators_headers (#209)

Six source modules each defined an identical 8-line _sc_headers(token)
function returning {"x-api-key": token, "Content-Type": "application/json"}.
Moved it to http.scrapecreators_headers() and migrated all 33 call sites.

Affected files: reddit.py, threads.py, tiktok.py, instagram.py, pinterest.py,
youtube_yt.py. Zero per-source variation, zero behavior change.

Net: -40 lines. 1022 tests pass (15 pre-existing failures unchanged).
Live smoke test: reddit search returns 12 threads with full engagement.

scripts/lib/instagram.py

@@ -112,14 +112,6 @@ def _log(msg: str):
     log.source_log("Instagram", msg)
 
 
-def _sc_headers(token: str) -> Dict[str, str]:
-    """Build ScrapeCreators request headers."""
-    return {
-        "x-api-key": token,
-        "Content-Type": "application/json",
-    }
-
-
 def _parse_date(item: Dict[str, Any]) -> Optional[str]:
     """Parse date from ScrapeCreators Instagram item to YYYY-MM-DD.
 
@@ -249,7 +241,7 @@ def _user_reels(
             from urllib.parse import urlencode
             params = urlencode({"handle": handle})
             url = f"{reels_url}?{params}"
-            headers = _sc_headers(token)
+            headers = http.scrapecreators_headers(token)
             headers["User-Agent"] = http.USER_AGENT
             data = http.get(url, headers=headers, timeout=30, retries=2)
         except Exception as e:
@@ -260,7 +252,7 @@ def _user_reels(
             resp = _requests.get(
                 reels_url,
                 params={"handle": handle},
-                headers=_sc_headers(token),
+                headers=http.scrapecreators_headers(token),
                 timeout=30,
             )
             resp.raise_for_status()
@@ -307,7 +299,7 @@ def search_instagram(
             from urllib.parse import urlencode
             params = urlencode({"query": core_topic})
             url = f"{SCRAPECREATORS_BASE}/v2/instagram/reels/search?{params}"
-            headers = _sc_headers(token)
+            headers = http.scrapecreators_headers(token)
             headers["User-Agent"] = http.USER_AGENT
             data = http.get(url, headers=headers, timeout=30, retries=2)
         except Exception as e:
@@ -318,7 +310,7 @@ def search_instagram(
             resp = _requests.get(
                 f"{SCRAPECREATORS_BASE}/v2/instagram/reels/search",
                 params={"query": core_topic},
-                headers=_sc_headers(token),
+                headers=http.scrapecreators_headers(token),
                 timeout=30,
             )
             resp.raise_for_status()
@@ -403,7 +395,7 @@ def fetch_captions(
             resp = _requests.get(
                 f"{SCRAPECREATORS_BASE}/v2/instagram/media/transcript",
                 params={"url": url},
-                headers=_sc_headers(token),
+                headers=http.scrapecreators_headers(token),
                 timeout=15,
             )
             if resp.status_code == 200: