[WIP] Refactor: right align metrics email

[MarceloRobert]

Readjusts the email formatting so that the larger number sections have a right-aligned text.

Changes

• Turned left-aligned text in metrics email's sections "COVERAGE" and "TEST LAB ACTIVITY" into right-aligned text;
• Adjusted spacing for some columns;
• Changed double space before change percentage into a single space (that's 123 (+99%) into 123 (+99%));
• Removed percentage sign if the metric diff is 0. This was done because many labs don't have tests, so the (+0%) can clutter the change column.
• WIP - use dynamic spacing in order to avoid large empty spaces on all-small numbers and also avoid lack of space for larger-than-usual numbers

How to test

• Have data in the database, I used /backend/tests_submissions/submissions.zip for testing
• Run poetry run python3 manage.py notifications --action metrics_summary and compare with the older version

Visual Reference

Example of new format:

KernelCI Metrics Summary
========================
Period: 2026-06-11 12:42 UTC to 2026-06-18 12:42 UTC


  COVERAGE
  --------
                          This week    Last week              Change
                          ─────────    ─────────           ─────────
  Trees monitored                 4            0                  +4
  Checkouts                      32            0                 +32
  Builds                        103            0                +103
  Tests                      15,720            0             +15,720

  BUILD REGRESSIONS
  -----------------
  A "regression" is defined as a reported problem affecting 0 or multiple builds.

  Origin    Regressions       Affected           Affected builds by top issues
            (known + new)     Builds (total)     #1      #2      #3
  ─────────────────────────────────────────────────────────────────────────
  maestro   0   + 1   = 1     1                  1       
  ─────────────────────────────────────────────────────────────────────────
  Total     0   + 1   = 1     1


  TOP REGRESSIONS PER ORIGIN
  --------------------------
  maestro:
    #1 (1 occurrences) -  in vmlinux (Makefile:1220) [logspec:kbuild,kbuild.other]
      Dashboard: https://d.kernelci.org/issue/maestro:5dd754a8374e1b44291f37f6a3510603dcbc6348?iv=1
  

  TEST LABS ACTIVITY
  ------------------
  13 labs reported results this week (13 more than last week).

  Labs marked with an asterisk (*) are new.

  Lab                     Covered builds    Boots        Tests      Change (tests)
  ────────────────────────────────────────────────────────────────────────────────
  lava-baylibre *                      1        1            0                   0
  lava-broonie *                      41        8        3,467              +3,467
  lava-cip *                           1        1            0                   0
  lava-clabbe *                        3        3            0                   0
  lava-collabora *                    82       29        1,805              +1,805
  lava-foundriesio *                   2        2            0                   0
  lava-kci-qualcomm *                  1        0            1                  +1
  lava-pengutronix *                   3        3            0                   0
  linaro *                           174      229       10,119             +10,119
  maestro *                            1        0            2                  +2
  microsoft *                          1        0            3                  +3
  opentest-ti *                        2        2            0                   0
  redhat *                             1        0           45                 +45
  ────────────────────────────────────────────────────────────────────────────────
  Total                              313      278       15,442             +15,442

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Closes #1938