chore(release): prepare v0.3.0 — Phase 3 MVP shipped (T306)

[Pratiyush]

Summary

• Moves the Phase 3 i18next JSON import/export entry from CHANGELOG [Unreleased] into [0.3.0] — 2026-04-19.
• Adds RELEASE-NOTES narrative for v0.3.0 — the headline is MVP shipped: Translately now works end-to-end without AI configured.
• Ticks Phase 3 tickets; flags T303 (async SSE) as moved-to-Phase-4.

After this merges, cut the signed v0.3.0 tag from master. That tag closes MVP.

Closes

• [T306] Tag v0.3.0 GPG-signed #57 T306 tag v0.3.0 GPG-signed — end of MVP

Test plan

• CHANGELOG + RELEASE-NOTES render locally
• bash scripts/gen-llms-txt.sh regens cleanly
• CI green (lint + lychee + codeql; no code in this PR)

Docs updated per CLAUDE.md rule #10

• Product / Architecture / API / Self-hosting — no surface change this PR.
• LLM-ingestible — docs/llms-full.txt regenerated.

Pre-merge

• GPG-signed commits
• Conventional Commits PR title
• One intent: tag v0.3.0
• I have read and accept the Contributor License Agreement.

Closes #57