docs: align v1.2.0 changelog, roadmap and release notes with shipped features

[btcneves]

Objective

Correct documentation inconsistencies introduced when the v1.2.0 release notes and roadmap were prepared before the log aggregation and notification provider extension commits landed on the branch.

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Changes

• CHANGELOG.md — adds log aggregation (Loki/Elasticsearch, Promtail, LOG_SINK) and notification provider extensions (PagerDuty dedup key, OpsGenie priority, custom payload template) to the v1.2.0 entry
• docs/roadmap.md — moves both features from the Future section into the v1.2.0 Completed table; replaces the now-empty Future table with a single-line note pointing to the issue tracker
• docs/release-notes-v1.2.0.md — adds two new sections: "Log aggregation" (direct-push and agent-based modes, label schema, docs/log-aggregation.md reference) and "Notification provider extensions" (dedup key, priority, payload template)
• backend/tests/conftest.py — sets JWT_SECRET_KEY to a 64-character test value, eliminating InsecureKeyLengthWarning from all 260 backend tests

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Test Plan

• uv run pytest — 260 backend tests pass, zero JWT warnings
• Review CHANGELOG v1.2.0 entry lists all six feature additions
• Review roadmap v1.2.0 Completed table has 8 rows; Future section is empty
• Review release notes contain "Log aggregation" and "Notification provider extensions" sections

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Security Checklist

• No real secrets, tokens, passwords or API keys in any committed file
• .env.production is not tracked
• All credentials masked in API responses, UI and delivery records
• No sensitive configuration details exposed in public documentation
• No git push --force, no history rewrite, no --no-verify bypass