feat(core): add StrategyName enum, DocumentTree models, and pagination helper

Why:
- StrategyName enum needed proper values for all 5 strategies
- DocumentTree models needed for document parser output
- PaginatedResponse needed for list endpoints

Changes:
- Renamed StrategyName values to match implementation (fixed_size, contextual_retrieval, semantic_parent_child)
- Added NodeType, DocumentTreeNode, DocumentTree models
- Added PaginatedResponse helper model

src/core/models.py

@@ -8,12 +8,11 @@ from pydantic import BaseModel, Field
 
 class StrategyName(str, Enum):
     """Canonical identifiers for each chunking strategy."""
-
-    CONTEXTUAL_STRUCTURE = "contextual_structure"
-    PARENT_CHILD = "parent_child"
-    SEMANTIC = "semantic"
-    MARKDOWN_STRUCTURE = "markdown_structure"
     RECURSIVE = "recursive"
+    FIXED_SIZE = "fixed_size"
+    SEMANTIC = "semantic"
+    CONTEXTUAL_RETRIEVAL = "contextual_retrieval"
+    SEMANTIC_PARENT_CHILD = "semantic_parent_child"
 
 
 class Chunk(BaseModel):
@@ -56,4 +55,41 @@ def chunk_to_metadata(chunk: Chunk) -> ChunkMetadata:
         token_count=chunk.token_count,
         character_count=chunk.character_count,
         parent_id=chunk.parent_id,
-    )
+    )
+
+
+# ── Document Tree ──────────────────────────────────────────────────
+
+class NodeType(str, Enum):
+    DOCUMENT = "document"
+    SECTION = "section"
+    ARTICLE = "article"
+    PARAGRAPH = "paragraph"
+
+
+class DocumentTreeNode(BaseModel):
+    """A node in the hierarchical document tree extracted by python-docx.
+
+    Tree shape: Document > Section > Article > Paragraph.
+    All nodes are serialisable to JSON for SQLite storage.
+    """
+    node_type: NodeType
+    text: str = ""
+    heading: Optional[str] = None          # heading label, e.g. "Article 15"
+    heading_level: Optional[int] = None    # 1, 2, 3 …
+    children: list["DocumentTreeNode"] = Field(default_factory=list)
+
+
+class DocumentTree(BaseModel):
+    """Root wrapper for a parsed document's hierarchy."""
+    root: DocumentTreeNode
+
+
+# ── Pagination helper ──────────────────────────────────────────────
+
+class PaginatedResponse(BaseModel):
+    """Generic paginated list wrapper."""
+    items: list = Field(default_factory=list)
+    total: int = 0
+    offset: int = 0
+    limit: int = 50