Dramatically improve EPG refresh performance by parsing the XML file once
per source instead of once per channel. The new implementation:
- Pre-filters to only process EPG entries mapped to actual channels
- Parses the entire XML file in a single pass
- Uses O(1) set lookups to skip unmapped channel programmes
- Skips non-mapped channels entirely with minimal overhead
For EPG sources with many channels but few mapped (e.g., 10,000 channels
with 100 mapped to channels), this provides approximately:
- 99% reduction in file open operations
- 99% reduction in XML file scans
- Proportional reduction in CPU and I/O overhead
The parse_programs_for_tvg_id() function is retained for single-channel
use cases (e.g., when a new channel is mapped via signals).
Fixes inefficient repeated file parsing that was occurring with large
EPG sources.
