docs/dev: sync backend rework, MQTT, and devcontainer hygiene

README: add Versioning (unified SemVer, pre-releases, build metadata); emphasize UTC handling and streaming endpoint; add Dev Container notes (UI-only Remote Containers, npm ci, idempotent aliases)
TECH-CHANGELOG: backend rework notes (serialization camelCase, UTC normalization, streaming metadata); add component build metadata template (image tags/SHAs)
Copilot instructions: integrate maintenance guardrails; reinforce UTC and camelCase conventions; document MQTT topics and scheduler retained payload behavior
Devcontainer: map Remote Containers to UI; remove in-container install; switch to npm ci; make aliases idempotent

README.md

@@ -217,6 +217,7 @@ For detailed deployment instructions, see:
  - Videos include a `video` payload with a stream URL and playback flags:
    - `video`: includes `url` (streaming endpoint) and `autoplay`, `loop`, `volume`, `muted`
    - Streaming endpoint supports byte-range requests (206) to enable seeking: `/api/eventmedia/stream/<media_id>/<filename>`
+   - Server-side UTC: All backend comparisons are performed in UTC; API returns ISO strings without `Z`. Frontend appends `Z` before parsing.
   
 ## Recent changes since last commit
 
@@ -232,6 +233,18 @@ These changes are designed to be safe if metadata extraction or probes fail —
 See `MQTT_EVENT_PAYLOAD_GUIDE.md` for details.
   - `infoscreen/{uuid}/group_id` - Client group assignment
 
+## 🧩 Developer Environment Notes (Dev Container)
+- Extensions: UI-only `Dev Containers` runs on the host UI; not installed inside the container to avoid reinstallation loops. See `/.devcontainer/devcontainer.json` (`remote.extensionKind`).
+- Installs: Dashboard uses `npm ci` on `postCreateCommand` for reproducible installs.
+- Aliases: `postStartCommand` appends shell aliases idempotently to prevent duplicates across restarts.
+
+## 📦 Versioning
+- Unified app version: Use a single SemVer for the product (e.g., `2025.1.0-beta.3`) — simplest for users and release management.
+- Pre-releases: Use identifiers like `-alpha.N`, `-beta.N`, `-rc.N` for stage tracking.
+- Build metadata: Optionally include component build info (non-ordering) e.g., `+api.abcd123,dash.efgh456,sch.jkl789,wkr.mno012`.
+- Component traceability: Document component SHAs or image tags under each TECH-CHANGELOG release entry rather than exposing separate user-facing versions.
+- Hotfixes: For backend-only fixes, prefer a patch bump or pre-release increment, and record component metadata under the unified version.
+
 ## 📁 Project Structure
 
 ```