feat(dupe-guard): add max_posts per-sub cap to prevent one-shot intro campaigns from re-posting

Adds max_posts INTEGER to campaign_subs (NULL = unlimited/evergreen).
Adds successful_post_count() query counting lifetime success records.
poster.py checks max_posts before the 7-day rolling dupe guard.

Root cause: campaign 2 fired 8 days after the last post (just outside the
7-day window), allowing a duplicate r/opensource pitch. Fix: set max_posts=1
on intro campaigns so the lifetime cap fires regardless of window.

app/db/migrations/018_campaign_subs_max_posts.sql

@@ -0,0 +1,2 @@
+-- Add per-sub post cap; NULL = unlimited (evergreen). Set max_posts=1 for one-shot intro campaigns.
+ALTER TABLE campaign_subs ADD COLUMN max_posts INTEGER;

app/db/store.py

@@ -319,6 +319,14 @@ class Store:
         )
         return row is not None
 
+    def successful_post_count(self, campaign_id: int, target: str) -> int:
+        """Return lifetime count of successful posts for this campaign+target pair."""
+        row = self._fetchone(
+            "SELECT COUNT(*) AS n FROM posts WHERE campaign_id = ? AND target = ? AND status = 'success'",
+            (campaign_id, target),
+        )
+        return row["n"] if row else 0
+
     # ------------------------------------------------------------------ #
     # Sub rules
     # ------------------------------------------------------------------ #

app/services/poster.py

@@ -53,6 +53,17 @@ def _run_post(db_path: str, campaign_id: int, target: str,
                 )
                 return {"skipped": True, "reason": f"occurrence {occurrence_str} not today"}
 
+        # Per-sub post cap (max_posts=None means unlimited)
+        max_posts = sub_row.get("max_posts")
+        if max_posts is not None:
+            count = store.successful_post_count(campaign_id, target)
+            if count >= max_posts:
+                logger.info(
+                    "Skipping %s / %s — reached max_posts=%d (posted %d time(s))",
+                    campaign_id, target, max_posts, count,
+                )
+                return {"skipped": True, "reason": f"max_posts={max_posts} reached for {target!r}"}
+
         # Dupe guard (opt-out allowed per strategy)
         if strategy.supports_dupe_guard() and store.already_posted_this_week(campaign_id, target):
             return {"skipped": True, "reason": f"already posted to {target!r} this week"}