feat(F-15): citation graph — PageRank, coupling, co-citation, CLI

- codex/graph.py: build_citation_graph (DiGraph from DB), citation_pagerank
  (graceful uniform fallback < 5 nodes), find_related (bibliographic coupling),
  find_co_cited, dangling_citations
- codex/config.py: graph_cite_boost_alpha=0.3, graph_min_corpus_size=15
- codex/cli.py: graph report [--top-n] [--json], graph related <paper-id>
  [--min-shared]; search paper --cite-boost (PageRank score weighting)
- tests/graph/: 39 tests (graph functions + CLI) — 326 total green

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

codex/config.py

@@ -286,6 +286,30 @@ class Settings(BaseSettings):
         ),
     )
 
+    # ------------------------------------------------------------------
+    # F-15 Literature Graph
+    # ------------------------------------------------------------------
+    graph_cite_boost_alpha: float = Field(
+        default=0.3,
+        ge=0.0,
+        le=1.0,
+        description=(
+            "Weight of the PageRank citation-boost in hybrid search. "
+            "Final score = dense_score * (1 + alpha * pagerank_score). "
+            "Set to 0.0 to disable the boost without removing the flag."
+        ),
+    )
+
+    graph_min_corpus_size: int = Field(
+        default=15,
+        gt=0,
+        description=(
+            "Minimum number of ingested papers for citation_pagerank to yield "
+            "meaningful rankings. Below this threshold a warning is logged and "
+            "uniform scores are returned."
+        ),
+    )
+
 
 @lru_cache(maxsize=1)
 def get_settings() -> Settings: