#1136 Add opt-in durable chat history, legacy import and browser compaction

Beschreibung

When the web app starts a new chat, owners need to keep previous conversations and import existing local histories without losing inactive branches. This adds an explicit local SQLite archive option, revision-safe chat saves, read-only legacy import, full backup, and browser prompt compaction that keeps the original conversation.

Draft for integration discussion. This rebuilds #727 on rewritten main after the history reset; it preserves the original contribution's author and the current request-body drain and forced-tool behavior.

- Opt in with `chat_archive_path` or `--chat-archive`; the default has no server-side persistent archive.
- Revision conflicts and write/capacity failures preserve the prior committed record. Imports are atomic, destination IDs win, and backups retain raw imported graphs and all branches.
- Archive routes retain API authentication and require the server's own host-and-port Origin, including when general API CORS is relaxed. Read-only `memory__search/recall` use existing MCP permissions and bounded message coordinates.
- Browser compaction persists original turns before summarizing older rounds, keeps recent rounds, and can restore the full context. Prompt counting uses the actual server encoder and tool schemas without running tools or inference.
- Legacy `/strata` and `/sw.js` compatibility retain old browser data.

Validation on native Windows/Python and Node:

- 40 archive, memory, archive HTTP and prompt-count tests passed.
- 319 affected server, security, MCP, frontend, lifecycle, settings, monitor, Responses and structured-output tests passed.
- 32 Node checks and 46 import assertions passed.
- Synthetic browser validation passed for generation, rename, starting a new chat while retaining the previous archive, and reload preserving the renamed archive list.
- Git diff check passed.

Limits: this is an opt-in archive for one server owner, with no per-account isolation. Draft persistence happens at chat save/actions rather than every keystroke. Browser validation uses a mock engine and synthetic database; Linux behavior and manual legacy-browser migration are not claimed as exercised by these Windows checks. Existing socket ResourceWarnings remain visible without test failures.

Independent review: one GA401 Antigravity/Gemini Flash/high round completed across four whole-file logical groups covering all 21 changed files. It reported no actionable findings. The Native Review owner revalidated the exact source binding and completed evidence. This is bounded review evidence; the reviewer did not execute tests, and split groups do not provide simultaneous attention to the full source.