chore: add Renovate config for automated dependency updates

[HemalathaMadeswaran18]

Closes #328

Purpose / Description

Dependencies are currently updated manually with no automated tooling to
detect outdated or vulnerable packages across the Python, Node.js, Docker,
GitHub Actions, and pre-commit ecosystems. This adds a Renovate Bot
configuration to automate dependency update PRs.

Fixes

• Fixes Add Renovate Bot for automated dependency updates #328

Approach

Added renovate.json at the repo root with the following behavior:

• Groups minor/patch updates into a single PR per ecosystem (Python,
  Node/pnpm, Docker, GitHub Actions, pre-commit) to reduce noise
• Major updates get individual PRs for careful review
• Patch-level updates automerge if CI passes
• Scheduled to run weekly on Saturdays to avoid disrupting active development
• Adds a Dependency label to all generated PRs

Covered package managers:

• Python: pep621 + uv (pyproject.toml, uv.lock)
• Node/pnpm: npm (web/package.json, pnpm-lock.yaml)
• Docker: dockerfile + docker-compose (docker/Dockerfile.api,
  docker/Dockerfile.web, docker/docker-compose.yml)
• GitHub Actions: .github/workflows/
• pre-commit: .pre-commit-config.yaml

How Has This Been Tested?

• Validated the JSON against the Renovate
  schema
• Config uses only documented options from the Renovate
  docs
• Full functionality will be verified once the Renovate GitHub App is
  installed on the repo — it will create a Dependency Dashboard issue confirming
  detection of all package managers

Learning (optional, can help others)

• Renovate's config:recommended preset handles most sensible defaults
  (pinning, range strategies, etc.)
• The matchManagers field scopes rules to specific ecosystems so grouping
  doesn't mix unrelated deps
• Renovate auto-detects all package files recursively — no need to specify
  paths manually
• Docs: https://docs.renovatebot.com/configuration-options/

Checklist

• All commits are signed off (git commit -s) per the
  DCO
• You have a descriptive commit message with a short title (first line,
  max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
  - [ ] UI changes: include screenshots of all affected screens (in particular
  showing any new or changed strings) and run any tests if necessary

[HemalathaMadeswaran18]

Note to Maintainer :
After this PR is merged, the Renovate GitHub App needs to be installed on the BlazeUp-AI org (scoped to the Observal repo). Once installed, Renovate will detect the existing config, skip the onboarding PR, and immediately start opening grouped dependency update PRs on the schedule defined here (weekly on Saturdays).