refactor: distribute Acaua as a Docker image instead of a desktop app

Acaua is a collective SOC daemon consuming only Zeek log files, not a
single-user tool needing local hardware like its siblings. Drop the
Tauri desktop shell, installers and self-update entirely; the server
and its web dashboard ship as a container, updated by pulling a new
image tag.

Link against the official prebuilt libduckdb instead of compiling it
from source via the bundled feature, cutting image build time from
15+ minutes to about 3. Fix a real bug found along the way: as PID 1
in a container, the daemon ignored SIGTERM and was always killed
after the stop timeout; it now shuts down gracefully.

Also corrects the Pillar 1 beaconing score: the period estimate had a
harmonic alias (a perfectly periodic 4h series resolved to 600s), and
the old score formula flagged pure random traffic as suspicious
(p95 = 0.839 on Poisson noise). The new formula was validated against
300 samples per scenario.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VAyr8d7HuAdUGB6aA5xmDD

CHANGELOG.md

@@ -6,6 +6,19 @@ Format based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); versio
 
 ## [Unreleased]
 
+### Changed
+- Distribution pivoted to 100% Docker and docker-compose (server daemon and web dashboard); native binary tarballs and desktop installers are discontinued.
+- DuckDB is now a mandatory runtime dependency, dynamically linked against the official prebuilt `libduckdb.so` (v1.5.6) with fixed SHA-256 via `scripts/fetch-libduckdb.sh`.
+- Upgraded workspace MSRV to Rust 1.88 (required by `duckdb ~1.10506.1`).
+- Upgraded container runtime and base image to Debian 13 trixie.
+
+### Fixed
+- Graceful shutdown on `SIGTERM` when running as PID 1 in container environments.
+
+### Removed
+- Tauri desktop application (`apps/gui/src-tauri`) and associated desktop build scripts.
+- Self-update flag (`acaua update --apply`) and in-place binary overwrite. Updates are now handled via image pulls (`docker compose pull`).
+
 ## [0.1.0] - 2026-10-31
 
 ### Added