rather large commit, it now handles memory and threading properly.

---

more thorough commit message which was generated by the wonderful copilot AI tool:

Refactor and enhance audio encoding and decoding pipelines

- Introduced CodecUtils.h for utility functions related to audio codec selection, including sample format and bitrate clamping.
- Added DecoderPipeline.h to manage audio decoding, including opening files and retrieving audio stream information.
- Implemented EncoderPipeline.h to handle audio encoding, including encoder setup, resampling, and writing output files.
- Created OutputPathBuilder.h to generate output file paths based on reencode job specifications.
- Refactored SpectrogramWidget to improve OpenGL handling and ensure safe context usage.
- Updated ReencodeDialog.h to enhance code readability and maintainability.
- Improved error handling and logging throughout the audio processing components.

src/engine/encoder/SampleRateUtils.h

@@ -6,15 +6,24 @@ extern "C" {
 
 namespace engine {
 
+/**
+ * @brief Selection Priority:
+ *  1. Exact match with sourceRate
+ *  2. Lowest supported rate that is >= sourceRate (upsample as little as possible)
+ *  3. Highest supported rate overall
+ *      rates are below sourceRate, eg a low rate phone codec at 8khz
+ */
 inline int pickSampleRate(const AVCodec *encoder, int sourceRate)
 {
     if (!encoder->supported_samplerates)
         return sourceRate;
 
+    // 1: exact match
     for (const int *r = encoder->supported_samplerates; *r != 0; ++r)
         if (*r == sourceRate)
             return sourceRate;
 
+    // 2: lowest rate >= sourceRate (minimal upsampling)
     int best = 0;
     for (const int *r = encoder->supported_samplerates; *r != 0; ++r) {
         if (*r >= sourceRate) {
@@ -25,6 +34,7 @@ inline int pickSampleRate(const AVCodec *encoder, int sourceRate)
     if (best != 0)
         return best;
 
+    // 3: all supported rates are below sourceRate
     for (const int *r = encoder->supported_samplerates; *r != 0; ++r)
         if (*r > best)
             best = *r;
@@ -32,4 +42,4 @@ inline int pickSampleRate(const AVCodec *encoder, int sourceRate)
     return best;
 }
 
-} // namespace engine
+} // namespace engine