refactor: drop requests dep, route all providers through lib/http urllib wrapper (#393)

Five provider modules (pinterest, threads, instagram, tiktok, youtube_yt)
and watchlist.py each carried a try/except `requests` import with parallel
urllib + requests branches. The urllib path already used the
stdlib-only wrapper at `lib/http.py` (retries, 429 handling, HTTPError).
This collapses every dual-branch into a single `http.get`/`http.post`
call and removes the `requests` dependency from `pyproject.toml`.

Also drops 4 transitive deps (urllib3, certifi, charset-normalizer, idna)
from the lockfile, leaving the skill stdlib-only at runtime.

Tests for tiktok comments and watchlist delivery were rewritten to mock
`lib.http` directly instead of the now-removed `requests` module.

Out of scope but flagged during review: the 13 surviving SC call sites
share a near-identical scaffold and would benefit from a
`http.scrapecreators_get(url, params, token, ...)` helper. Filed for a
follow-up PR rather than expanding scope here.

skills/last30days/scripts/watchlist.py

@@ -10,16 +10,11 @@ import sys
 import time
 from pathlib import Path
 
-try:
-    import requests
-except ImportError:
-    requests = None
-
 SCRIPT_DIR = Path(__file__).parent.resolve()
 sys.path.insert(0, str(SCRIPT_DIR))
 
 import store
-from lib import schema
+from lib import http, schema
 
 
 # --- Webhook Delivery Functions ---
@@ -58,34 +53,21 @@ def _format_delivery_message(topic: str, counts: dict, mode: str) -> str:
 
 def _send_slack_webhook(url: str, text: str) -> None:
     """POST to Slack incoming webhook."""
-    if not requests:
-        raise RuntimeError("requests library not available for webhook delivery")
-    
-    response = requests.post(
-        url,
-        json={"text": text},
-        headers={"Content-Type": "application/json"},
-        timeout=10,
-    )
-    response.raise_for_status()
+    http.post(url, json_data={"text": text}, timeout=10, retries=1)
 
 
 def _send_generic_webhook(url: str, text: str) -> None:
     """POST JSON payload to generic webhook."""
-    if not requests:
-        raise RuntimeError("requests library not available for webhook delivery")
-    
-    response = requests.post(
+    http.post(
         url,
-        json={
+        json_data={
             "message": text,
             "source": "last30days",
             "timestamp": time.time(),
         },
-        headers={"Content-Type": "application/json"},
         timeout=10,
+        retries=1,
     )
-    response.raise_for_status()
 
 
 # --- Command Handlers ---