feat(active): add the active beers page, run metadata and tap numbers

Adds the pages that were missing: an "Active Beers" view of what is in the
fermenter right now, and the per-run metadata (canonical name, tap number,
original gravity, brew notes) that could not be derived from the readings.

Why the metadata is needed
--------------------------
The `beer_data` collection stores a beer name on *each reading*, and that
name drifts within a run - run 4 was entered as "Necatron-Christmas Beer",
then "Nocturna the Christmas beer", then "Nocturna Christmas beer". There was
nowhere to record a tap number, a confirmed original gravity, or a note about
the brew.

The new `beer_runs` collection holds one document per run with exactly those
details. It starts empty; `scripts/backfill_run_metadata.py` populates it
from the existing readings. The script is idempotent, only ever inserts into
`beer_runs`, never touches `beer_data`, and never overwrites a name you have
already set by hand.

The active beers page
---------------------
A card per brew that is still in the fermenter, showing the tap number, the
current gravity, the projected alcohol, how many days it has been brewing, a
sparkline of the gravity so far, and the latest comment on a reading.

A run with only one reading and no recorded original gravity has no reliable
estimate, so the alcohol figure shows a dash rather than a misleading number
- record the OG or take a second reading and it appears. The same applies to
the existing reads, where the legacy app's assumed OG of 1012 produced
*negative* alcohol for a perfectly normal OG.

Also adds the history page: alcohol by brew, brews per month, every run in a
table, a comparison of the standard ABV formula against the legacy one, and
a list of readings that fall outside the plausible hydrometer range.

Changes to the existing pages
-----------------------------
- The brew-process page and the run detail page now show the canonical run
  name and the latest comment, and the run detail page lets you edit the
  metadata inline.
- Gravity is displayed as a decimal reading (1.039) with the point scale
  beneath it (39 points), which is the way a brewer reads it. Labelling the
  stored `1039` as "1039 points" would have been wrong.
- A run's display name now prefers the name on its *final* reading - the one
  written when the brew was finished, so the name the operator settled on -
  falling back to the most-used name.
- `upsert_run_metadata` now clears a field when it is passed as `None`, so a
  name or tap number can be deleted. `upsert_run_metadata_partial` is the
  non-clearing variant, used by the backfill.

Bugs found and fixed along the way
----------------------------------
- The chart script was included both in the base template and again by each
  page, so every chart was drawn twice. A test now asserts it is loaded
  exactly once per page.
- The chart library was never referenced by the base template, so all charts
  were silently dead. That was fixed in the previous PR.

Tests
-----
91 (up from 74). New coverage: the active page contents, the tap number, the
projected alcohol, the "needs 2nd reading" case, metadata saving and
clearing, the stored name overriding the drifting reading name, the next free
tap number, the partial-update semantics, the history page, the outlier
list, and the point-scale formatting.

Data safety
-----------
`beer_data` is untouched by this change. The backfill script only inserts into
`beer_runs` and skips any run whose name has already been set by hand.

tests/test_brewing.py

@@ -184,17 +184,36 @@ def test_run_is_active_until_a_final_reading_exists():
     assert final_reading(finished.readings) is not None
 
 
-def test_run_display_name_prefers_the_most_used_label():
-    """The stored beer_type drifts across readings; the most-used one wins."""
+def test_run_display_name_prefers_the_name_on_the_final_reading():
+    """The stored beer_type drifts across readings.  The name on the final
+    reading is the one the operator settled on."""
     run = Run.from_docs("4", [
         _doc(date="2024-11-23", beer_type="Necatron-Christmas Beer"),
         _doc(date="2024-11-28", beer_type="Nocturna the Christmas beer"),
         _doc(date="2024-11-30", beer_type="Nocturna Christmas beer", final="True"),
     ])
-    # All three are unique, so the most recent wins.
     assert run.display_name() == "Nocturna Christmas beer"
 
 
+def test_run_display_name_falls_back_to_the_most_used_label():
+    """Without a final reading, the most-used name wins, breaking ties
+    towards the most recent."""
+    run = Run.from_docs("5", [
+        _doc(date="2024-12-03", beer_type="Going Nowhere Fast"),
+        _doc(date="2024-12-08", beer_type="Going Nowhere fast", final="True"),
+    ])
+    # The final reading's name wins.
+    assert run.display_name() == "Going Nowhere fast"
+
+    run2 = Run.from_docs("6", [
+        _doc(date="2024-12-03", beer_type="Balter"),
+        _doc(date="2024-12-06", beer_type="Pale ale blayer"),
+        _doc(date="2024-12-10", beer_type="Balter"),
+    ])
+    # No final reading; "Balter" was used twice.
+    assert run2.display_name() == "Balter"
+
+
 def test_run_display_name_falls_back_to_the_run_number():
     run = Run.from_docs("7", [_doc(run_id="7", beer_type="", date="2026-01-01")])
     assert run.display_name() == "Run 7"
@@ -305,3 +324,21 @@ def test_run_summary_handles_an_empty_run():
     assert summary["abv"] is None
     assert summary["reading_count"] == 0
     assert summary["name"] == "Run 99"
+
+
+def test_point_scale_is_the_digits_after_the_decimal():
+    """In brewing convention 1.039 is 39 points.  The stored 1039 is the full
+    reading and must not be labelled as a point count."""
+    run = Run.from_docs("1", [
+        _doc(date="2026-01-01", sg="1039"),
+    ])
+    summary = run_summary(run)
+    assert summary["min_sg_decimal"] == "1.039"
+    assert summary["min_sg_point_scale"] == "39"
+
+
+def test_point_scale_for_a_round_reading():
+    run = Run.from_docs("2", [_doc(date="2026-01-01", sg="1010")])
+    summary = run_summary(run)
+    assert summary["min_sg_point_scale"] == "10"
+    assert summary["min_sg_decimal"] == "1.010"