fix: build the image on Python 3.14, and fix the review findings

The image build failed on master:

    ERROR: Ignored the following versions that require a different python
    version: 4.19.0 Requires-Python >=3.11
    ERROR: Could not find a version that satisfies the requirement
    pymongo==4.19.0

The base image was `python:3.10` but the pinned pymongo 4.19 needs Python
>= 3.11. The image is now `python:3.14-slim` — the current stable release,
so it is not carrying the CVEs fixed in 3.11, 3.12 and 3.13.

All the runtime dependencies ship a cp314 wheel (pymongo included), so no
compiler toolchain is needed. The old apt-get line installed
libsasl2-dev, python-dev-is-python3, libldap2-dev and libssl-dev for
packages that are no longer in the dependency list, so it is gone and the
image is smaller.

The build now asserts the interpreter version, so a base image change that
breaks the pinned requirements fails the build with a clear message rather
than a pip resolution error.

Review findings, all from the independent review of this PR
------------------------------------------------------------
- Mongo indexes: the run metadata is always looked up by `beer_run_id` and
  the readings by `(beer_run_id, date)`. Both are now indexed.
- Content-Security-Policy: the app serves only its own vendored assets, so
  a strict `default-src 'self'` policy is enforceable. Adding it required
  removing the inline `style=` attributes from the templates (they became
  `.mb-lg` / `.mt-lg` utility classes) and the one inline `<script> (the run
  id now travels as a `data-run-id` attribute). A test asserts no template
  has either.
- Job timeouts: the `test` and `build` jobs now have `timeout-minutes`, so
  a wedged build stops occupying a runner.
- OG range: raised from 1300 to 1500 points, so a strong specialty brew
  (a 1.150 wort is real) can be recorded.

Bugs found and fixed along the way
----------------------------------
- An unfinished run reported a *measured* ABV. `abv_percent()` now returns
  None for a run that is still in the fermenter — its figure is a projection,
  not a measurement. This was showing the two active runs in the "alcohol by
  brew" chart as if they were finished.
- The `/api/runs` endpoints the charts fetch from did not exist, so every
  chart was silently empty. They are back, with tests.
- The "brews over time" chart left out the months with no brews, which
  spaced the busy months evenly and made the chart read as continuous
  activity. The quiet months are now included with a zero.
- The run-detail chart was dropped when PR 1 was scoped down, leaving dead
  code behind. It is restored.

Tests: 105 (up from 91).

src/flask/templates/history.html

@@ -14,7 +14,7 @@
     </div>
 </div>
 
-<div class="grid grid--4" style="margin-bottom:1.25rem">
+<div class="grid grid--4 mb-lg">
     <div class="stat">
         <div class="stat__label">Total brews</div>
         <div class="stat__value">{{ stats.total_runs }}</div>
@@ -45,7 +45,7 @@
     </div>
 </div>
 
-<div class="grid grid--2" style="margin-bottom:1.25rem">
+<div class="grid grid--2 mb-lg">
     <section class="card">
         <div class="card__head">
             <h2 class="card__title">Alcohol by brew</h2>
@@ -59,7 +59,7 @@
     <section class="card">
         <div class="card__head">
             <h2 class="card__title">Brews over time</h2>
-            <span class="card__hint">Runs started per month</span>
+            <span class="card__hint">Runs started per month (active runs counted at their first reading)</span>
         </div>
         <div class="card__body">
             <canvas id="months-chart" height="120"></canvas>
@@ -68,7 +68,7 @@
 </div>
 
 {% if outliers %}
-<section class="card" style="margin-bottom:1.25rem">
+<section class="card mb-lg">
     <div class="card__head">
         <h2 class="card__title">Readings worth a second look</h2>
         <span class="card__hint">{{ outliers | length }} outside the usual hydrometer range</span>
@@ -102,7 +102,7 @@
 </section>
 {% endif %}
 
-<section class="card" style="margin-bottom:1.25rem">
+<section class="card mb-lg">
     <div class="card__head">
         <h2 class="card__title">Every brew</h2>
         <span class="card__hint">{{ summaries | length }} runs</span>