tz: server-wide display timezone via TZ env var (default EST/EDT)

User-reported issue: server logs were timestamped in UTC ("05:36:20"
when local was ~01:36 EDT), and the PDF report's "Created" footer
similarly showed raw UTC.  Inconsistent with the modal which already
converts to browser local via toLocaleString.

Solution: standard Linux TZ env var.  Set once in the container, and:
  - Python's datetime.now() uses local
  - Logging module's timestamps use local
  - matplotlib renderers + report_pdf formatters use local
  - astimezone() conversions resolve to the configured TZ

DB columns stay UTC (created_at uses SQLite's strftime('%Y-...Z', 'now')
which is always UTC, regardless of TZ env var — proper "store UTC,
display local" pattern).

Changes:
  - Dockerfile: install tzdata (python:3.11-slim omits the timezone
    database), set default TZ=America/New_York
  - sfm/report_pdf.py: _fmt_iso_to_bw and _split_iso_to_date_time now
    convert UTC inputs (Z-suffixed) to local via astimezone(); naïve
    inputs (BW recorded-at, already unit-local) returned as-is.
    New _to_display_local helper centralizes the logic.
  - "Created" line in the PDF page footer now uses the converted
    timestamp.

Override per-deployment via the TZ env var in docker-compose
(separate commit on terra-view side).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -2,10 +2,21 @@ FROM python:3.11-slim
 
 WORKDIR /app
 
+# tzdata is required for the TZ env var to take effect (python:slim
+# omits the timezone database).  Without it, datetime.now() / logging
+# / matplotlib all stay in UTC regardless of TZ.  Default zone gets
+# set further down via ENV; users override per-deployment via the
+# `TZ` env var in docker-compose.
 RUN apt-get update && \
-    apt-get install -y --no-install-recommends curl && \
+    apt-get install -y --no-install-recommends curl tzdata && \
     rm -rf /var/lib/apt/lists/*
 
+# Default display timezone — applied to server logs, datetime.now(),
+# matplotlib rendered timestamps, and any naïve-vs-aware datetime
+# conversions in the PDF renderer.  Override via TZ env var in
+# docker-compose; storage in the DB is always UTC regardless.
+ENV TZ=America/New_York
+
 COPY pyproject.toml requirements.txt ./
 COPY minimateplus ./minimateplus
 COPY micromate    ./micromate