Phase 1: PeerTube plugin and recon_domains module

Single source of truth for the 18 RECON knowledge domains mapped to
PeerTube category IDs 100-117. Replaces duplicate VALID_DOMAINS sets
in enricher.py and embedder.py with imports from lib/recon_domains.py.

Includes PeerTube plugin (peertube-plugin-recon-domains) that registers
custom categories via videoCategoryManager.addConstant(), and a parity
test to verify constants match between RECON and the PeerTube API.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

peertube-plugin/peertube-plugin-recon-domains/README.md

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+# peertube-plugin-recon-domains
+
+Registers 18 RECON knowledge domains as PeerTube video categories using IDs 100-117. These categories are assigned automatically by RECON's domain assignment pipeline based on concept extraction analysis.
+
+## Category Mapping
+
+| ID  | Domain                    |
+|-----|---------------------------|
+| 100 | Agriculture & Livestock   |
+| 101 | Civil Organization        |
+| 102 | Communications            |
+| 103 | Food Systems              |
+| 104 | Foundational Skills       |
+| 105 | Logistics                 |
+| 106 | Medical                   |
+| 107 | Navigation                |
+| 108 | Operations                |
+| 109 | Power Systems             |
+| 110 | Preservation & Storage    |
+| 111 | Security                  |
+| 112 | Shelter & Construction    |
+| 113 | Technology                |
+| 114 | Tools & Equipment         |
+| 115 | Vehicles                  |
+| 116 | Water Systems             |
+| 117 | Wilderness Skills         |
+
+Built-in PeerTube categories (IDs 1-18) are not modified.
+
+## Install
+
+```bash
+# Copy plugin to PeerTube storage
+cp -r peertube-plugin-recon-domains /var/www/peertube/storage/plugins/node_modules/
+
+# Register plugin via API or admin UI
+# Admin > Plugins > Install > peertube-plugin-recon-domains
+
+# Restart PeerTube
+sudo systemctl restart peertube
+```
+
+## Uninstall
+
+Remove the plugin via PeerTube admin UI or:
+
+```bash
+rm -rf /var/www/peertube/storage/plugins/node_modules/peertube-plugin-recon-domains
+sudo systemctl restart peertube
+```
+
+Videos with RECON categories will revert to showing the raw category ID until the plugin is reinstalled.