fix(analysis): the review findings - rising gravity, undated readings, the band

A second, deeper review of PR #12 found five things the first review did
not.  All are fixed here.

The band was not the band
-------------------------
The fermentation curve's shaded range was drawn as ONE dataset with
`fill: "+1"`, which fills to the NEXT dataset - and the next dataset was
the median line, not the maximum.  So the band shaded min-to-median, the
upper half of the range was drawn as a hairline but never shaded, and the
tooltip said "range min to max" while the picture showed something else.
The band is now two datasets - the min curve filled down to a max curve -
with the max hidden from the legend.

Rising gravity
--------------
`attenuation_percent` rejected a final gravity below 1.000 and one
implying more than 100% attenuation, but a run whose gravity went UP
produced a NEGATIVE attenuation - which was then averaged into the
headline figure and plotted as a negative bar.  Verified against the live
code: OG 1010 -> FG 1020 gave -100.0%.  Now rejected, and reported by
`implausible_runs` with the reason "the gravity went up, which is usually
a mis-ordered pair of readings".

Undated readings
----------------
An undated reading sorted to the front of the run, so it became the
original gravity - which put the OG after the FG and produced a negative
attenuation.  The OG now comes from the earliest *dated* reading, and
`days_to_target` skips the readings it cannot place on the timeline rather
than giving up on the whole run.  Verified: readings 1050 (dated), 1005
(undated), 1010 (dated) gave None instead of 24 days, and an OG of 1005
instead of 1050.

Active runs on the curve
------------------------
The curve chart filtered on `curve.length > 1` only, so it plotted the
active runs as well - a partial fermentation on a chart meant to describe
how a completed one behaves.  It now excludes them, along with the
suspect runs.

Four fetches, one payload
-------------------------
`/api/runs` carries every reading of every run, and four charts each
fetched it independently - so the analysis page made two identical
round-trips of the whole collection.  There is now one shared fetch.

Also
----
- The "Readings that need a second look" table links to the run, so a
  suspect reading can be corrected in one click.
- `days_to_1015` and the `curves` variable were computed and never used.
  Removed.
- A reading below 1.000 now says what it is - "beer cannot be less dense
  than water, so this is almost certainly a typo (a 1.012 reading entered
  as 012?)".  The reading is still SAVED: rejecting it would lose the
  brewer's actual observation, and seeing it recorded is how they spot the
  mistake.

142 tests (up from 134).

src/flask/views/brews.py

@@ -49,6 +49,10 @@ class ReadingForm(FlaskForm):
     beer_run_type = StringField("Beer / cider name",
                                 validators=[Optional(), Length(max=120)])
     Date = DateField("Reading date", validators=[DataRequired()])
+    # The floor is deliberately lower than is_plausible_sg's.  An
+    # out-of-range reading is SAVED with a warning rather than rejected:
+    # the brewer may have mis-typed it, and seeing it recorded is how they
+    # spot and correct the mistake.
     sg = IntegerField("Gravity reading (points)",
                       validators=[DataRequired(), NumberRange(min=900, max=1500)])
     final_reading = BooleanField("This is the final reading")
@@ -82,11 +86,24 @@ def index():
     if form.validate_on_submit():
         sg_points = int(form.sg.data)
         if not is_plausible_sg(sg_points):
-            flash(
-                f"{sg_points} is outside the usual hydrometer range "
-                "(990-1200). It was saved anyway — double-check the reading.",
-                "warning",
-            )
+            if sg_points < 1000:
+                # Beer cannot be less dense than water, so this is always
+                # a mis-entry - and a mis-typed "1.012" as "012" is the
+                # likely cause.  It is still saved so the mistake is
+                # visible and can be corrected.
+                flash(
+                    f"{sg_points} is below 1.000 — beer cannot be less dense "
+                    "than water, so this is almost certainly a typo (a 1.012 "
+                    "reading entered as 012?). It was saved anyway; correct "
+                    "it on the run's page.",
+                    "warning",
+                )
+            else:
+                flash(
+                    f"{sg_points} is outside the usual hydrometer range "
+                    "(990-1200). It was saved anyway — double-check the reading.",
+                    "warning",
+                )
         repo().upsert_reading(
             beer_run_id=str(form.beer_run_id.data),
             beer_type=(form.beer_run_type.data or "").strip(),