Config param placeholders - #193

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# Conversation that produced these changes
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## User prompt

"the placeholders for introducing custom commands should be surrounded by "{}" @airflow_src/dags/impl/processor_impl.py#L254-264"

## Clarifying round 1

Q: Existing settings in the DB use bare placeholders (RAW_FILE_PATH). After the switch to {RAW_FILE_PATH} they silently stop substituting. How should this be handled?
  - Fail loudly on leftovers
  - Clean break, no guard  <-- chosen
  - Accept both syntaxes

Q: Braces have to be allowed through check_for_malicious_content. Which scope?
  - New allow_placeholders flag
  - Add {} to the with-spaces pattern
  - Other  <-- chosen: "validate after substitution only"

## User prompt

"decision 2: the webapp should know which parameters exists and to a "dummy substitution" before validating"

## Clarifying round 2

Q: Where should the placeholder registry live, and should the webapp help list be generated from it?
  - New shared/config_params.py, generated help  <-- chosen
  - New module, hand-written help
  - Put it in shared/keys.py

## User prompt

"check again that all your desired changes are there"

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>