feat: replace legacy taxonomy and tag logic with facets

[Flatts3000]

Replaced the outdated taxonomy system with facet-based filtering (topics, feelings, meanings) stored as post meta. Removed TaxonomyRoute and TaxonomyService classes. Added migrations for facets and embeddings, enhancing semantic search capabilities. Updated related services to leverage facet-based logic and introduced the EmbeddingService for similarity matching. Refined attachment sync and metadata processing workflows for better integration.

[Copilot]

Pull Request Overview

This PR replaces the legacy taxonomy and tag system with a facet-based approach for categorizing PostSecret content. The system now uses three distinct facet types (topics, feelings, meanings) stored as post meta, enabling more semantic search capabilities and better AI classification.

• Migrated from single tag-based taxonomy to structured facets (topics, feelings, meanings)
• Added embedding service for semantic similarity matching using OpenAI embeddings
• Updated AI prompt and schema to generate facet-based classifications instead of generic tags

Reviewed Changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
single-secret.php	Updates template to display facets organized by type instead of generic tags
parts/card.php	Modifies card display to show combined facets with type-aware styling
SchemaGuard.php	Updates schema validation to handle facets and removes handwriting field
Prompt.php	Major refactor of AI prompt to generate structured facets instead of tags
Metadata.php	Adds color filtering for palette generation using perceptual distance
Ingress.php	Updates result storage to save facets separately as post meta
EmbeddingService.php	New service for generating and managing text embeddings for semantic search
AttachmentSync.php	Updates sync logic to use combined facets for captions
AdminMetaBox.php	Enhances admin interface to display facets by type with re-classify functionality
postsecret-ai.php	Adds embedding generation to processing workflow and re-classify action
TaxonomyService.php	Removes legacy taxonomy service (no longer needed)
SearchService.php	Updates search to use facet-based filtering instead of tags
ModerationService.php	Adds facet management methods for content moderation
TaxonomyRoute.php	Removes legacy taxonomy REST route
Secret.php	Updates model to use facets instead of tags
run-migrations.php	Adds migration runner script
postsecret-admin.php	Removes TaxonomyRoute registration
003_embeddings.php	Creates embeddings table for semantic search
002_facets.php	Migration to remove tag alias table

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