refactor: reorganize folder structure for better maintainability

Changes:
- Move migrations/ under open_notebook/database/migrations/
- Extract AI models to open_notebook/ai/ (Model, ModelManager, provision)
- Extract podcasts to open_notebook/podcasts/ (EpisodeProfile, SpeakerProfile, PodcastEpisode)
- Reorganize prompts to mirror graphs structure (chat/, source_chat/)

This improves code organization by:
- Consolidating database concerns (migrations now with database code)
- Separating AI infrastructure from domain entities
- Isolating podcast feature into its own module
- Creating consistent prompt/graph naming conventions

All 52 tests pass.

open_notebook/podcasts/models.py

@@ -0,0 +1,148 @@
+from typing import Any, ClassVar, Dict, List, Optional, Union
+
+from pydantic import Field, field_validator
+from surrealdb import RecordID
+
+from open_notebook.database.repository import ensure_record_id, repo_query
+from open_notebook.domain.base import ObjectModel
+
+
+class EpisodeProfile(ObjectModel):
+    """
+    Episode Profile - Simplified podcast configuration.
+    Replaces complex 15+ field configuration with user-friendly profiles.
+    """
+
+    table_name: ClassVar[str] = "episode_profile"
+
+    name: str = Field(..., description="Unique profile name")
+    description: Optional[str] = Field(None, description="Profile description")
+    speaker_config: str = Field(..., description="Reference to speaker profile name")
+    outline_provider: str = Field(..., description="AI provider for outline generation")
+    outline_model: str = Field(..., description="AI model for outline generation")
+    transcript_provider: str = Field(
+        ..., description="AI provider for transcript generation"
+    )
+    transcript_model: str = Field(..., description="AI model for transcript generation")
+    default_briefing: str = Field(..., description="Default briefing template")
+    num_segments: int = Field(default=5, description="Number of podcast segments")
+
+    @field_validator("num_segments")
+    @classmethod
+    def validate_segments(cls, v):
+        if not 3 <= v <= 20:
+            raise ValueError("Number of segments must be between 3 and 20")
+        return v
+
+    @classmethod
+    async def get_by_name(cls, name: str) -> Optional["EpisodeProfile"]:
+        """Get episode profile by name"""
+        result = await repo_query(
+            "SELECT * FROM episode_profile WHERE name = $name", {"name": name}
+        )
+        if result:
+            return cls(**result[0])
+        return None
+
+
+class SpeakerProfile(ObjectModel):
+    """
+    Speaker Profile - Voice and personality configuration.
+    Supports 1-4 speakers for flexible podcast formats.
+    """
+
+    table_name: ClassVar[str] = "speaker_profile"
+
+    name: str = Field(..., description="Unique profile name")
+    description: Optional[str] = Field(None, description="Profile description")
+    tts_provider: str = Field(
+        ..., description="TTS provider (openai, elevenlabs, etc.)"
+    )
+    tts_model: str = Field(..., description="TTS model name")
+    speakers: List[Dict[str, Any]] = Field(
+        ..., description="Array of speaker configurations"
+    )
+
+    @field_validator("speakers")
+    @classmethod
+    def validate_speakers(cls, v):
+        if not 1 <= len(v) <= 4:
+            raise ValueError("Must have between 1 and 4 speakers")
+
+        required_fields = ["name", "voice_id", "backstory", "personality"]
+        for speaker in v:
+            for field in required_fields:
+                if field not in speaker:
+                    raise ValueError(f"Speaker missing required field: {field}")
+        return v
+
+    @classmethod
+    async def get_by_name(cls, name: str) -> Optional["SpeakerProfile"]:
+        """Get speaker profile by name"""
+        result = await repo_query(
+            "SELECT * FROM speaker_profile WHERE name = $name", {"name": name}
+        )
+        if result:
+            return cls(**result[0])
+        return None
+
+
+class PodcastEpisode(ObjectModel):
+    """Enhanced PodcastEpisode with job tracking and metadata"""
+
+    table_name: ClassVar[str] = "episode"
+
+    name: str = Field(..., description="Episode name")
+    episode_profile: Dict[str, Any] = Field(
+        ..., description="Episode profile used (stored as object)"
+    )
+    speaker_profile: Dict[str, Any] = Field(
+        ..., description="Speaker profile used (stored as object)"
+    )
+    briefing: str = Field(..., description="Full briefing used for generation")
+    content: str = Field(..., description="Source content")
+    audio_file: Optional[str] = Field(
+        default=None, description="Path to generated audio file"
+    )
+    transcript: Optional[Dict[str, Any]] = Field(
+        default_factory=dict, description="Generated transcript"
+    )
+    outline: Optional[Dict[str, Any]] = Field(
+        default_factory=dict, description="Generated outline"
+    )
+    command: Optional[Union[str, RecordID]] = Field(
+        default=None, description="Link to surreal-commands job"
+    )
+
+    class Config:
+        arbitrary_types_allowed = True
+
+    async def get_job_status(self) -> Optional[str]:
+        """Get the status of the associated command"""
+        if not self.command:
+            return None
+
+        try:
+            from surreal_commands import get_command_status
+
+            status = await get_command_status(str(self.command))
+            return status.status if status else "unknown"
+        except Exception:
+            return "unknown"
+
+    @field_validator("command", mode="before")
+    @classmethod
+    def parse_command(cls, value):
+        if isinstance(value, str):
+            return ensure_record_id(value)
+        return value
+
+    def _prepare_save_data(self) -> dict:
+        """Override to ensure command field is always RecordID format for database"""
+        data = super()._prepare_save_data()
+        
+        # Ensure command field is RecordID format if not None
+        if data.get("command") is not None:
+            data["command"] = ensure_record_id(data["command"])
+            
+        return data