MM-5C: Finalize model and metadata contract

[Atharva-Kanherkar]

Parent issue: #16

MM-5C: Finalize model and metadata contract

Problem

The base model already has some multimodal fields, but the write/read contract is not fully standardized and emotional_profile is missing.

Repo impact

• models/base.py
• models/episodic.py
• stores/semantic_store.py
• stores/episodic_store.py
• serialization code in api/app.py

Tasks

• keep modality defaulting to "text"
• standardize allowed modality values: text, image, audio, video, multimodal
• preserve media_ref and media_type
• add emotional_profile: dict[str, float] to EpisodicMemory
• serialize nested emotional profile safely for Chroma metadata
• decide whether source_mime_type remains separate or is collapsed into media_type
• add has_media derivation for events

Acceptance criteria

• semantic and episodic records round-trip the new fields
• plain text records still default to modality="text" and media_ref=None
• emotional profile round-trips without violating Chroma metadata constraints

Risks

• Chroma metadata is primitive-only, so dict storage needs JSON or flattening