Fix/epg collision

[sparrowjack63]

Problem

The EPG guide displays the same programme on multiple unrelated channels
when the provider returns a non-unique epg_channel_id.

Some Xtream panels populate epg_channel_id poorly: dozens of unrelated
channels share a single placeholder value (the same id repeated across many
different channels). Each of these channels is correctly resolved to a
distinct external XMLTV xmltv_channel_id, yet the guide shows one
channel's programmes on all of them.

Root cause

getResolvedProgrammes() fetches programmes correctly (via the unique
xmltvChannelId for external, channelId for provider-native), but stores
them in the result map keyed by epgChannelId:

val lookupKey = channel.epgChannelId?.trim()?.takeIf(String::isNotEmpty)
    ?: channel.streamId.takeIf { it > 0L }?.toString()
    ?: continue
result[lookupKey] = progs

When several channels share the same epgChannelId, they collide on the same
map entry — the last one processed wins, and every other channel inherits its
guide. The grid reads back with the same epgChannelId-first key
(Channel.guideLookupKey()), so the collision is consistent end to end.

Reproduction

1. Provider returns the same epg_channel_id for several unrelated channels.
2. Each channel is resolved to a distinct EXTERNAL xmltv_channel_id.
3. Guide shows the same programme for all of them.

Example: two unrelated channels (say channel A and channel B) each map to
their own XMLTV entry with different programmes, but because both carry the
same provider epg_channel_id, the guide shows channel B's programme on
channel A as well.

Setting channels.epg_channel_id = NULL (without touching
channel_epg_mappings) immediately fixes the guide — isolating the key
collision as the cause.

Fix

Prefer streamId (always unique per channel within a provider) as the guide
lookup key, in both the producer (EpgResolutionEngine.getResolvedProgrammes,
external + provider blocks), the consumer (Channel.guideLookupKey), and the
playback path (getResolvedProgramsForPlaybackChannel).

epgChannelId is still used for provider-native programme joins
(ProgramDao.getForChannelsSync joins on channel_id = epgChannelId), so:

• no schema change, no migration,
• provider-native guide behaviour is preserved,
• the change only affects the identity key used to bucket programmes per
  channel, never the lookup key used to fetch them.

Testing

• ./gradlew testDebugUnitTest passes.
• Verified on a real device: with the provider's duplicate epg_channel_id
  values intact in the DB, each channel now shows its own distinct programme.
  Before the patch, they shared one guide.

Fixes #210.