🐛 Fix API key OPDS links in v1.2 feed

[aaronleopold]

Fixes #516

The feed generation logic was missing some key changes to properly route users to the correct location when using API keys in the OPDS v1.2 feed. The logic had to be made a bit more dynamic to account for the two scenarios:

1. No API key
2. API key

[codecov]

Codecov Report

Attention: Patch coverage is 17.47212% with 222 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
apps/server/src/routers/opds/v1_2.rs	0.00%	115 Missing ⚠️
core/src/opds/v1_2/feed.rs	0.00%	48 Missing ⚠️
core/src/opds/v1_2/entry.rs	0.00%	45 Missing ⚠️
apps/server/src/middleware/auth.rs	77.04%	14 Missing ⚠️

Files with missing lines	Coverage Δ
apps/server/src/middleware/auth.rs	92.16% <77.04%> (-1.35%)	⬇️
core/src/opds/v1_2/entry.rs	41.72% <0.00%> (-1.72%)	⬇️
core/src/opds/v1_2/feed.rs	50.28% <0.00%> (+16.43%)	⬆️
apps/server/src/routers/opds/v1_2.rs	0.00% <0.00%> (ø)