feat(pipeline): structural metadata end-to-end (Blocks D2-D4)

D2: RAG service stores section/section_title/paragraph/paragraph_num/page
from embedding service chunks_with_metadata into Qdrant payloads.

D3: Control generator prefers section > article > section_title from
Qdrant, adds page to source_citation and generation_metadata.

D4: Validated with real BGB §§ 312-312k text. Found and fixed critical
bug where Phase 3 overlap destroyed the [§ ...] section prefix, causing
only the first chunk per document to have metadata. All subsequent
chunks lost section info.

Also fixes pre-existing lint issues (unused imports, ambiguous variable
names, duplicate dict key, bare except).

456 tests passing (58 embedding + 387 pipeline + 11 rag-service).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

control-pipeline/services/decomposition_pass.py

@@ -24,7 +24,6 @@ import json
 import logging
 import os
 import re
-import uuid
 from dataclasses import dataclass, field
 from typing import Optional
 
@@ -56,7 +55,7 @@ ANTHROPIC_API_URL = "https://api.anthropic.com/v1"
 # Patterns are defined in normative_patterns.py and imported here
 # with local aliases for backward compatibility.
 
-from .normative_patterns import (
+from .normative_patterns import (  # noqa: E402
     PFLICHT_RE as _PFLICHT_RE,
     EMPFEHLUNG_RE as _EMPFEHLUNG_RE,
     KANN_RE as _KANN_RE,
@@ -3472,7 +3471,7 @@ class DecompositionPass:
                 "category": atomic.category,
                 "parent_uuid": parent_uuid,
                 "gen_meta": json.dumps({
-                    "decomposition_source": candidate_id,
+                    "decomposition_source_id": candidate_id,
                     "decomposition_method": "pass0b",
                     "engine_version": "v2",
                     "action_object_class": getattr(atomic, "domain", ""),
@@ -4104,6 +4103,8 @@ def _format_citation(citation) -> str:
                     parts.append(c["article"])
                 if c.get("paragraph"):
                     parts.append(c["paragraph"])
+                if c.get("page") is not None:
+                    parts.append(f"S. {c['page']}")
                 return " ".join(parts) if parts else citation
         except (json.JSONDecodeError, TypeError):
             return citation