Document the event pipeline and expose priority constants

[loevgaard]

Fixes #22

Stacked on #41.

Problem

The bundle is a pipeline of ten listeners on ConversionsApiEventRaised whose order lived as magic numbers spread across ten files, documented nowhere. An integrator could not tell that a listener at the default priority is the intended place to enrich the event, that everything is already populated by then, or that a listener below -850 may never run because a filter stopped propagation.

Change

No behaviour change. Every priority keeps the exact value it had.

• Four constants on ConversionsApiEventRaised name the bands: PRIORITY_POPULATE, PRIORITY_ENRICH, PRIORITY_FILTER and PRIORITY_SEND. Every subscriber now derives its priority from them, so there is one source of truth.
• The class docblock carries the table.
• README.md gains a How it works section with the full pipeline, the two rules that follow from it, and a worked enrichment listener that attaches the logged-in customer's email, name and external id.

Tests

Five unit tests pin the contract down: the pipeline is in descending order, the four populate listeners run above PRIORITY_ENRICH, the filters and sinks run below it, and the two sinks sit at PRIORITY_SEND.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.79%. Comparing base (1d50dd3) to head (3bc77c7).

Additional details and impacted files

@@                       Coverage Diff                       @@
##             fix/21-pixel-access-token      #42      +/-   ##
===============================================================
+ Coverage                        67.07%   67.79%   +0.72%     
  Complexity                         135      135              
===============================================================
  Files                               30       30              
  Lines                              413      413              
===============================================================
+ Hits                               277      280       +3     
+ Misses                             136      133       -3     

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