Add per-paper summary verification and cap safeguards

- Per-paper summary batches are fact-checked against abstracts (verify ->
  revise once on major issues -> re-verify), controlled by SEBRIEF_VERIFY_ENABLED
- The paper cap never drops papers already published in this week's digest
  (--force re-runs could otherwise erase them from every digest)
- SEBRIEF_MAX_PAPERS default 40 -> 60 (observed weekly volume ~47)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/_fakes.py

@@ -138,6 +138,7 @@ class FakeStore:
     def __init__(self, *, pending: list[Paper] | None = None) -> None:
         self.upserted: list[list[Paper]] = []
         self.saved_summaries: list[dict] = []
+        self.published_dois: set[str] = set()
         self.pending = pending if pending is not None else []
         self.digests: dict[str, object] = {}
         self.digested: list[tuple[list[str], str]] = []
@@ -157,6 +158,9 @@ class FakeStore:
     def mark_digested(self, dois: list[str], week: str) -> None:
         self.digested.append((list(dois), week))
 
+    def digested_dois(self, week: str) -> set[str]:
+        return set(self.published_dois)
+
     def has_digest(self, week: str) -> bool:
         return week in self.digests
 
@@ -198,6 +202,8 @@ class FakeProvider:
         fail_summarize: bool = False,
         fail_revise: bool = False,
         fail_paper_summaries: bool = False,
+        paper_verifications: list | None = None,
+        revised_papers: list | None = None,
     ) -> None:
         self.digest = digest or make_digest()
         self.verifications = list(verifications or [])
@@ -205,6 +211,8 @@ class FakeProvider:
         self.fail_summarize = fail_summarize
         self.fail_revise = fail_revise
         self.fail_paper_summaries = fail_paper_summaries
+        self.paper_verifications = list(paper_verifications or [])
+        self.revised_papers = revised_papers
         self.calls: list[str] = []
 
     def summarize(self, payload):
@@ -227,6 +235,20 @@ class FakeProvider:
         ]
         return items, "fake-model"
 
+    def verify_paper_summaries(self, papers, items):
+        self.calls.append("verify_papers")
+        if not self.paper_verifications:
+            from sebrief.models import Verification
+
+            return Verification(ok=True, issues=[]), "fake-model"
+        return self.paper_verifications.pop(0), "fake-model"
+
+    def revise_paper_summaries(self, papers, items, issues):
+        self.calls.append("revise_papers")
+        if self.revised_papers is not None:
+            return list(self.revised_papers), "fake-model"
+        return list(items), "fake-model"
+
     def verify(self, payload, digest):
         self.calls.append("verify")
         if not self.verifications: