feat(error-log): add a concise error logging capability

[efahl]

Record build errors in a rotating log to allow easier diagnosis of upstream imagebuilder and package build issues. Add a separate web page for access to the log's content. Log content is exposed via an API call, so other downstream tools may utilize the data.

Note that the log content is intentionally minimal and anonymized to minimize any use for tracking ASU users.

---

Web page from my tests:

Total errors logged: 431 (showing 10 most recent)
Time period: 2025-10-16T17:04 - 2025-10-20T13:24

2025-10-20T09:00 24.10-SNAPSHOT:ath79/generic:elecom_wrc-300ghbk2-i 409 Client Error: Conflict (can only kill running containers. e03fcd1e72a08f3b15501ec3b93e48245c868c1556eeb17fa93809c90123225d is in state stopped: container state improper)
2025-10-20T12:29 24.10-SNAPSHOT:ath79/generic:elecom_wrc-300ghbk2-i Impossible package selection
2025-10-20T12:33 24.10-SNAPSHOT:x86/64:generic Impossible package selection
2025-10-20T12:34 SNAPSHOT:x86/64:generic Impossible package selection
2025-10-20T12:59 SNAPSHOT:x86/64:generic Impossible package selection: bort
2025-10-20T13:01 SNAPSHOT:x86/64:generic Impossible package selection: dnsmasq not in manifest
2025-10-20T13:04 24.10-SNAPSHOT:x86/64:generic Impossible package selection: dnsmasq-full
2025-10-20T13:15 24.10-SNAPSHOT:x86/64:generic Impossible package selection: dnsmasq-full invalid
2025-10-20T13:21 24.10-SNAPSHOT:ath79/generic:tplink_archer-c7-v4 Impossible package selection: base-files invalid
2025-10-20T13:24 24.10-SNAPSHOT:ath79/generic:elecom_wrc-300ghbk2-i Impossible package selection: base-files invalid

[codecov[bot]]

Codecov Report

:x: Patch coverage is 97.14286% with 2 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 92.55%. Comparing base (5e65dec) to head (bb9995a). :warning: Report is 312 commits behind head on main.

Files with missing lines	Patch %	Lines
asu/build.py	75.00%	1 Missing :warning:
asu/main.py	75.00%	1 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1522       +/-   ##
===========================================
+ Coverage   80.75%   92.55%   +11.79%     
===========================================
  Files          15       15               
  Lines         977     1451      +474     
===========================================
+ Hits          789     1343      +554     
+ Misses        188      108       -80     

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[efahl]

Example use of the API:

$ curl -s 'https://my-asu-server.lan/api/v1/build-errors?n_entries=50' | awk 'NR>3 {print $2}' | sort | uniq -c | sort -nr
     24 24.10-SNAPSHOT:ath79/generic:elecom_wrc-300ghbk2-i
     19 24.10-SNAPSHOT:x86/64:generic
      4 SNAPSHOT:x86/64:generic
      2 24.10-SNAPSHOT:ath79/generic:tplink_archer-c7-v4
      1 24.10.1:lantiq/xrx200:bt_homehub-v5a