feat(F-16): chunk quality gate + section classification

- codex/quality.py: 3-signal filter (length / alpha-ratio / bib-score)
  + rule-based section classifier + run_quality_pass retroactive DB pass
- codex/ingest.py: promote quality imports to module level; apply
  filter_chunks before embedding; store section column in chunks INSERT
- codex/config.py: CHUNK_MIN_CHARS / CHUNK_MIN_ALPHA_RATIO / CHUNK_MAX_BIB_SCORE
- infra/schema.sql: ALTER TABLE chunks ADD COLUMN IF NOT EXISTS section TEXT
- .env.example: document F-16 quality thresholds
- codex/cli.py: quality run sub-command (scope by --paper-id or all papers)
- tests/quality/: 35 new tests covering all quality functions
- tests/ingest/: patch filter_chunks in source-path tests to isolate ingest

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

codex/config.py

@@ -191,9 +191,7 @@ class Settings(BaseSettings):
 
     synthesis_llm_url: str | None = Field(
         default=None,
-        description=(
-            "Ollama base URL for synthesis. When None, falls back to OLLAMA_BASE_URL."
-        ),
+        description=("Ollama base URL for synthesis. When None, falls back to OLLAMA_BASE_URL."),
     )
 
     synthesis_top_k: int = Field(
@@ -256,6 +254,38 @@ class Settings(BaseSettings):
         ),
     )
 
+    # ------------------------------------------------------------------
+    # F-16 Chunk Quality Gate
+    # ------------------------------------------------------------------
+    chunk_min_chars: int = Field(
+        default=60,
+        gt=0,
+        description=(
+            "Minimum character count for a chunk to pass the quality gate. "
+            "Chunks shorter than this value are discarded before embedding."
+        ),
+    )
+
+    chunk_min_alpha_ratio: float = Field(
+        default=0.40,
+        ge=0.0,
+        le=1.0,
+        description=(
+            "Minimum fraction of alphabetic characters in a chunk. "
+            "Chunks with a lower ratio are treated as OCR artefacts and discarded."
+        ),
+    )
+
+    chunk_max_bib_score: float = Field(
+        default=0.70,
+        ge=0.0,
+        le=1.0,
+        description=(
+            "Maximum bibliography heuristic score before a chunk is rejected. "
+            "Score is based on DOI density, 'et al.' and year-in-brackets patterns."
+        ),
+    )
+
 
 @lru_cache(maxsize=1)
 def get_settings() -> Settings: