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/reddit.py

@@ -69,14 +69,6 @@ def _log(msg: str):
     log.source_log("Reddit", msg, tty_only=False)
 
 
-def _sc_headers(token: str) -> Dict[str, str]:
-    """Build ScrapeCreators request headers."""
-    return {
-        "x-api-key": token,
-        "Content-Type": "application/json",
-    }
-
-
 def _extract_core_subject(topic: str) -> str:
     """Extract core subject from verbose query.
 
@@ -335,7 +327,7 @@ def _global_search(
     try:
         data = http.get(
             f"{SCRAPECREATORS_BASE}/search",
-            headers=_sc_headers(token),
+            headers=http.scrapecreators_headers(token),
             params={"query": query, "sort": sort, "timeframe": timeframe},
             timeout=30,
             retries=2,
@@ -373,7 +365,7 @@ def _subreddit_search(
     try:
         data = http.get(
             f"{SCRAPECREATORS_BASE}/subreddit/search",
-            headers=_sc_headers(token),
+            headers=http.scrapecreators_headers(token),
             params={
                 "subreddit": subreddit,
                 "query": query,
@@ -405,7 +397,7 @@ def fetch_post_comments(
     try:
         data = http.get(
             f"{SCRAPECREATORS_BASE}/post/comments",
-            headers=_sc_headers(token),
+            headers=http.scrapecreators_headers(token),
             params={"url": url},
             timeout=30,
             retries=2,