Manifest fetch across the configured servers (JR-025 … JR-037)

Satisfies JRay-public-server UR-007. Servers are tried in configured order and
the first result clearing the configured tier wins; first-match rather than
best-match because querying every server for every item multiplies egress and
leaks the library to more parties, and the ordering already encodes which
source the admin prefers.

Every server is untrusted, including the pre-configured community one, so a
fetched manifest is re-validated against the same rules the server applies on
upload: envelope version refused if unknown, identifiers format-checked,
windows bounds-checked against the *local* file's runtime, belief bounded to
[0, 1], control and bidi characters refused in names. Responses are capped
while streaming rather than after buffering, since a hostile server can declare
any Content-Length it likes. HTTPS is required away from loopback. A failing
server is skipped with exponential backoff so one dead server cannot stall a
sweep.

The audio-tier offset is applied once, at store time, so stored truth is always
in the local file's own timebase and no read path needs offset awareness.
Windows are shifted, never reshaped — merging adjacent ones would answer "was a
face visible" rather than "was the actor present" (SR-002).

Also records why there is no `exact` tier, which was missing and led me to
re-add one. The file-hash tier is withdrawn on legal grounds: a TMDB id
discloses "some copy of this film", but an OpenSubtitles hash discloses "this
exact release", which turns a catalogue lookup into a release-identification
service and a server's database into a mapping from file fingerprints to the
instances holding them. The reason now lives on MatchTier and in SPEC.md §JR-036,
`TitleQuery` has no VideoHash property so there is nothing to send, and a test
asserts the enum has no Exact member — the spec had still listed `exact` as a
configurable tier, which is what made the removal look like an oversight.

42 tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

TRACES: JR-025, JR-027, JR-028, JR-029, JR-030, JR-031, JR-036, JR-037 | PR-005, PR-006

Jellyfin.Plugin.JRay/Models/JmanifestScene.cs

@@ -0,0 +1,38 @@
+using System.Collections.ObjectModel;
+using System.Text.Json.Serialization;
+
+namespace Jellyfin.Plugin.JRay.Models;
+
+/// <summary>
+/// One presence window.
+/// </summary>
+/// <remarks>
+/// <b>A window is a claim about scene membership, not a recognition event</b>
+/// (SR-002). An actor who turns away, is occluded, or is off-camera while the
+/// shot cuts to whoever they are speaking to is still present — so a consumer
+/// must never read a window boundary as "the face was detected here", and must
+/// not merge, split or trim windows.
+/// </remarks>
+public class JmanifestScene
+{
+    /// <summary>Gets or sets the window start, in seconds.</summary>
+    [JsonPropertyName("start")]
+    public double Start { get; set; }
+
+    /// <summary>Gets or sets the window end, in seconds, inclusive.</summary>
+    [JsonPropertyName("end")]
+    public double End { get; set; }
+
+    /// <summary>
+    /// Gets or sets the accumulated posterior that justified this claim, in [0, 1].
+    /// </summary>
+    [JsonPropertyName("belief")]
+    public double? Belief { get; set; }
+
+    /// <summary>
+    /// Gets or sets how the actor was identified: <c>live</c>, <c>deferred</c>
+    /// or <c>pooled</c>.
+    /// </summary>
+    [JsonPropertyName("route")]
+    public string? Route { get; set; }
+}