Task 4.5: Update Pydantic models to use key as primary identifier

- Updated AnimeSeriesResponse and SearchResult models in anime.py:
  - Changed 'id' field to 'key' as the primary series identifier
  - Added 'folder' as optional metadata field
  - Added field validator to normalize key to lowercase and strip whitespace
  - Added comprehensive docstrings explaining identifier usage

- Updated DownloadItem and DownloadRequest models in download.py:
  - Added field validator for serie_id normalization (lowercase, stripped)
  - Improved documentation for serie_id (primary identifier) vs serie_folder (metadata)

- Updated test_anime_models.py with comprehensive tests:
  - Tests for key normalization and whitespace stripping
  - Tests for folder as optional metadata
  - Reorganized tests into proper class structure

- Updated test_download_models.py with validator tests:
  - Tests for serie_id normalization in DownloadItem
  - Tests for serie_id normalization in DownloadRequest

All 885 tests pass.

src/server/models/anime.py

@@ -1,9 +1,24 @@
+"""Anime Pydantic models for the Aniworld web application.
+
+This module defines request/response models used by the anime API
+and services. Models are focused on serialization, validation, 
+and OpenAPI documentation.
+
+Note on identifiers:
+- key: Primary identifier (provider-assigned, URL-safe, e.g., 'attack-on-titan')
+- folder: Filesystem folder name (metadata only, e.g., 'Attack on Titan (2013)')
+"""
 from __future__ import annotations
 
+import re
 from datetime import datetime
 from typing import List, Optional
 
-from pydantic import BaseModel, Field, HttpUrl
+from pydantic import BaseModel, Field, HttpUrl, field_validator
+
+
+# Regex pattern for valid series keys (URL-safe, lowercase with hyphens)
+KEY_PATTERN = re.compile(r'^[a-z0-9][a-z0-9-]*[a-z0-9]$|^[a-z0-9]$')
 
 
 class EpisodeInfo(BaseModel):
@@ -31,10 +46,30 @@ class MissingEpisodeInfo(BaseModel):
 
 
 class AnimeSeriesResponse(BaseModel):
-    """Response model for a series with metadata and episodes."""
+    """Response model for a series with metadata and episodes.
+    
+    Note on identifiers:
+    - key: Primary identifier (provider-assigned, URL-safe, e.g., 'attack-on-titan')
+      This is the unique key used for all lookups and operations.
+    - folder: Filesystem folder name (metadata only, e.g., 'Attack on Titan (2013)')
+      Used only for display and filesystem operations.
+    """
 
-    id: str = Field(..., description="Unique series identifier")
+    key: str = Field(
+        ...,
+        description=(
+            "Series key (primary identifier) - provider-assigned URL-safe "
+            "key (e.g., 'attack-on-titan'). Used for lookups/identification."
+        )
+    )
     title: str = Field(..., description="Series title")
+    folder: Optional[str] = Field(
+        None,
+        description=(
+            "Series folder name on disk (metadata only) "
+            "(e.g., 'Attack on Titan (2013)'). For display/filesystem ops only."
+        )
+    )
     alt_titles: List[str] = Field(default_factory=list, description="Alternative titles")
     description: Optional[str] = Field(None, description="Short series description")
     total_episodes: Optional[int] = Field(None, ge=0, description="Declared total episode count if known")
@@ -42,20 +77,56 @@ class AnimeSeriesResponse(BaseModel):
     missing_episodes: List[MissingEpisodeInfo] = Field(default_factory=list, description="Detected missing episode ranges")
     thumbnail: Optional[HttpUrl] = Field(None, description="Optional thumbnail image URL")
 
+    @field_validator('key', mode='before')
+    @classmethod
+    def normalize_key(cls, v: str) -> str:
+        """Normalize key to lowercase."""
+        if isinstance(v, str):
+            return v.lower().strip()
+        return v
+
 
 class SearchRequest(BaseModel):
     """Request payload for searching series."""
 
-    query: str = Field(..., min_length=1)
-    limit: int = Field(10, ge=1, le=100)
-    include_adult: bool = Field(False)
+    query: str = Field(..., min_length=1, description="Search query string")
+    limit: int = Field(10, ge=1, le=100, description="Maximum number of results")
+    include_adult: bool = Field(False, description="Include adult content in results")
 
 
 class SearchResult(BaseModel):
-    """Search result item for a series discovery endpoint."""
+    """Search result item for a series discovery endpoint.
+    
+    Note on identifiers:
+    - key: Primary identifier (provider-assigned, URL-safe, e.g., 'attack-on-titan')
+      This is the unique key used for all lookups and operations.
+    - folder: Filesystem folder name (metadata only, e.g., 'Attack on Titan (2013)')
+      Used only for display and filesystem operations.
+    """
 
-    id: str
-    title: str
-    snippet: Optional[str] = None
-    thumbnail: Optional[HttpUrl] = None
-    score: Optional[float] = None
+    key: str = Field(
+        ...,
+        description=(
+            "Series key (primary identifier) - provider-assigned URL-safe "
+            "key (e.g., 'attack-on-titan'). Used for lookups/identification."
+        )
+    )
+    title: str = Field(..., description="Series title")
+    folder: Optional[str] = Field(
+        None,
+        description=(
+            "Series folder name on disk (metadata only) "
+            "(e.g., 'Attack on Titan (2013)'). For display/filesystem ops only."
+        )
+    )
+    snippet: Optional[str] = Field(None, description="Short description or snippet")
+    thumbnail: Optional[HttpUrl] = Field(None, description="Thumbnail image URL")
+    score: Optional[float] = Field(None, ge=0.0, le=1.0, description="Search relevance score (0-1)")
+
+    @field_validator('key', mode='before')
+    @classmethod
+    def normalize_key(cls, v: str) -> str:
+        """Normalize key to lowercase."""
+        if isinstance(v, str):
+            return v.lower().strip()
+        return v