chore(release): consolidate v1.5.1–v1.5.4 into single v1.5.0 Unreleased entry

None of v1.5.1 / v1.5.2 / v1.5.3 / v1.5.4 ever shipped — they only
existed as separate CHANGELOG entries on this unmerged feature
branch.  SemVer ties version numbers to releases, not commits.
From a field machine's perspective, the world skips straight from
v1.4.4 to whatever the next built-and-pushed installer is tagged.

Revert VERSION / AppVersion / module-docstring version comments
to v1.5.0 across:
  - series3_watcher.py     (VERSION = "1.5.0")
  - installer.iss          (AppVersion=1.5.0)
  - series3_tray.py        (docstring)
  - settings_dialog.py     (docstring)
  - README.md              (banner + footer)

Consolidate the four split CHANGELOG sections into a single
"## [Unreleased] — v1.5.0" entry covering all the work in one
release.  Configuration key table consolidated into one place.
No commit history rewriting — the per-commit code changes still
make sense as logical units for code review; only the surface-
level version metadata + CHANGELOG narrative was misleading.

Going forward: accumulate work-in-progress under an "[Unreleased]"
heading, bump VERSION only at actual release time.

series3_watcher.py

@@ -247,7 +247,7 @@ def scan_latest(
 
 
 # --- API heartbeat / SFM telemetry helpers ---
-VERSION = "1.5.4"
+VERSION = "1.5.0"
 
 
 def _read_log_tail(log_file: str, n: int = 25) -> Optional[list]: